link: add tests for zero args and existing destination

[anihal]

Adds two edge-case tests for link:

• test_link_no_arguments: verifies that calling link with no arguments fails with the expected usage error
• test_link_dest_exists: verifies that attempting to hard-link when the destination already exists fails with "File exists"

The existing tests cover nonexistent source, circular links, and wrong argument counts (1 or 3), but these two cases were missing.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/misc/sleep is no longer failing!
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!
Congrats! The gnu test tests/tail/tail-n0f is now passing!
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/csplit/csplit-heap is now passing!
Congrats! The gnu test tests/printf/printf-surprise is now passing!
Congrats! The gnu test tests/tail/pipe-f is now passing!

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cut/bounded-memory. tests/cut/bounded-memory is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/date/resolution. tests/date/resolution is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/misc/io-errors is no longer failing!

[sylvestre]

thanks for these tests but they are failing for now

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Congrats! The gnu test tests/expand/bounded-memory is now passing!
Congrats! The gnu test tests/tail/pipe-f is now passing!

[anihal]

Hi @sylvestre, the failing tests have been fixed:

• test_link_dest_exists — broadened stderr_contains("File exists") to "exists" since OpenBSD reports "Already exists" for EEXIST
• test_link_no_arguments — removed the stderr_contains assertion since clap emits a different message for 0 args vs 1 arg across platforms
• Rustfmt — collapsed the method chain to one line

All 8 link tests now pass across platforms. The 3 currently failing CI jobs (test_mkfifo, test_stdbuf, test_timeout) are pre-existing flaky failures unrelated to this PR.

Could you take another look? Thanks!

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cut/bounded-memory. tests/cut/bounded-memory is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/misc/io-errors is no longer failing!
Congrats! The gnu test tests/basenc/bounded-memory is now passing!
Congrats! The gnu test tests/dd/no-allocate is now passing!

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/date/resolution. tests/date/resolution is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/basenc/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/misc/io-errors is no longer failing!
Congrats! The gnu test tests/tail/retry is no longer failing!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/misc/io-errors is no longer failing!
Congrats! The gnu test tests/tail/retry is no longer failing!

[codspeed-hq]

Merging this PR will degrade performance by 3.41%

❌ 1 regressed benchmark
✅ 293 untouched benchmarks
⏩ 42 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	cp_large_file[16]	379.7 µs	393.1 µs	-3.41%

---

_{Comparing anihal:link-edge-case-tests (585d31e) with main (bed3108)}

Footnotes

1. 42 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/misc/io-errors is no longer failing!
Congrats! The gnu test tests/tail/retry is no longer failing!
Note: The gnu test tests/cut/cut-huge-range is now being skipped but was previously passing.