test: Honor PATHEXT on Windows

[lhecker]

test was hardcoding the executable extensions on Windows.
This commit hooks it up to the PATHEXT env variable.

[Copilot]

🟢 Approval recommended

No unresolved review comments remain, and all requested changes are covered.

Pull request overview

Updates Windows test -x behavior to honor the PATHEXT environment variable.

Changes:

• Parse PATHEXT with a fallback to default executable extensions.
• Add Windows coverage for custom and excluded extensions.

File summaries

File	Description
tests/by-util/test_test.rs	Adds Windows regression coverage for custom PATHEXT behavior.
src/uu/test/src/platform/windows.rs	Uses PATHEXT for executable-extension matching.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)