fix: align unit tests with system Perl behavior

[fglock]

Summary

Three fixes to align jperl behavior with system Perl, resolving unit test failures found via prove src/test/resources/unit:

• $\ interpolation in double-quoted strings: Removed backslash from the non-interpolating characters set in StringDoubleQuoted.java so "$" is recognized as the output record separator variable, matching Perl's greedy variable capture
• sysread on in-memory handles: Changed IOOperator.java to return undef with "Bad file descriptor" for sysread on PerlIO::scalar handles, matching system Perl behavior
• IPC::Open3 stderr merging: Fixed IPCOpen3.java isUsableHandle() to check truthiness (not just definedness) of the error handle parameter. Per IPC::Open3 docs, a false $err (including "") means stderr merges with stdout

Test fixes

• string_interpolation.t: expect "$\" to equal "" (not "")
• sysread_syswrite.t: expect sysread on in-memory handle to return undef
• ipc_open3.t: fix tests that used $err="" expecting separate stderr; use gensym for fileno test, expect merged output for empty-string tests

All three test files now produce identical pass/fail results on both system Perl and jperl.

Test plan

• All 14 ipc_open3.t tests pass on both system Perl and jperl
• string_interpolation.t passes on both
• sysread_syswrite.t passes on both
• make passes (build + all unit tests)
• prove src/test/resources/unit - all tests successful

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