test(xtask): cover xtask Cargo unit tests

[ErichDonGubler]

No description provided.

[cwfitzgerald]

This 100% should be covered by cargo nextest...

[cwfitzgerald]

Seems to work for me on trunk?

 Nextest run ID e8207c28-32de-4292-a183-ee9d65ceea60 with nextest profile: default
    Starting 1 test across 32 binaries (2477 tests skipped)
        PASS [   0.009s] wgpu-xtask::bin/wgpu-xtask blah
────────────
     Summary [   0.404s] 1 test run: 1 passed, 2477 skipped
[INFO  wgpu_xtask::test] Finished tests

[ErichDonGubler]

@cwfitzgerald: Ah, I see this in the Test job CI logs, now:

wgpu-xtask::bin/wgpu-xtask util::test_git_version_parsing

Feeling silly, but hey, that's life. 😅