fix: use StaticNetworkUnikernelIP constant in httpStaticNetTest

[mdryaan]

Description

httpStaticNetTest in tests/e2e/test_functions.go derived the unikernel's IP address by splitting the tap device's IPv4 and hardcoding .2 as the last octet. constants.StaticNetworkUnikernelIP (172.16.1.2) already defines this value.

This removes the tap interface lookup, address iteration, and IP recomputation entirely, replacing them with a direct use of the constant. Also removes the commented-out net/http block (lines 420–434) that was never resolved. The
now-unused imports "net" and "github.com/asaskevich/govalidator" are dropped; "github.com/urunc-dev/urunc/internal/constants" is added.

Related issues

• Fixes httpStaticNetTest hardcodes unikernel IP offset and contains unresolved dead code #66

How was this tested?

tests/e2e/test_functions.go is the only file changed. The tap-finding and IP derivation block (lines 371–407) is replaced with
url := fmt.Sprintf("http://%s:8080", constants.StaticNetworkUnikernelIP). The commented-out block (lines 420–434) is removed.

golangci-lint run -v --timeout=5m — 0 new issues.
make unittest — all pass.
make — builds cleanly.

Full e2e verification requires a Firecracker + Unikraft environment running the
make — builds cleanly.

Full e2e verification requires a Firecracker + Unikraft environment running the
Firecracker-unikraft-httpreply-static-net test case.

LLM usage

N/A

Checklist

• I have read the contribution guide.
• The linter passes locally (make lint).
• The e2e tests of at least one tool pass locally (make test_ctr, make test_nerdctl, make test_docker, make test_crictl).
• If LLMs were used: I have read the llm policy.