test(git): enhance commit signing tests

[sisp]

Changes

I've factored out some test enhancements related to commit signing per @viceice's request (#29550 (comment)) from #29550 as a preparatory step for merging that PR.

Context

#29550 will add support for SSH-based commit signing in addition to existing GPG-based commit signing. For improved test robustness with two signing methods, I extended some existing tests there with more specific mocks and additional assertions. But to avoid mixing concerns and potential regressions in #29550, I've factored out the changes to GPG-related tests here.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[viceice]

Test's are failing on macos

[sisp]

I had accidentally hardcoded /tmp/ as the temporary directory in the tests, which doesn't seem to work on macOS (and likely not on Windows either). I've fixed it in 1f0ce9a.

[renovate-release]

🎉 This issue has been resolved in version 38.7.1 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 38.7.1

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