test-macos-3 fails a lot

[twpayne]

Describe the bug

chezmoi's CI runs a bunch of tests. The test-macos-3 test often (~50% of the time) fails with a timeout after 10 minutes, but passes when the test is re-run. For an example failure, see https://github.com/twpayne/chezmoi/actions/runs/13487705954/job/37681029733.

To reproduce

I cannot reproduce this, but see "Additional Context" below.

Expected behavior

chezmoi's CI should pass reliably.

Output of command with the --verbose flag

n/a

Output of chezmoi doctor

n/a. This has been happening for a while.

Additional context

At this point, I have only a few observations:

• The problem did not occur a few months ago, and I don't believe that the problem is related to the CI infrastructure. It's always the test-macos-3 job that fails with a timeout. If it were related to the CI infrastructure then I would expect all test-macos-* jobs to fail equally often.
• Each time, the job fails at a different point. There is no consistency.
• Looking at the logs, tests that should be really fast (i.e. take less than a second) instead take several seconds to run. Clearly, there's something wrong with this set of tests. If it were a simple "noisy neighbor" (i.e. overloaded CI) problem, then all jobs would fail equally often.
• I've made various efforts to tweak the macOS test sharding (by reducing the last first letter of the test), but it's consistently test-macos-3 that fails.
• Running some kind of git bisect on the test shards would likely find the problem, but it would take someone with a lot of patience to run this.

cc @KapJI because you've helped so much in the past and might have insight here.