[3.12] gh-106584: Fix exit code for unittest in Python 3.12 (GH-106588)

[miss-islington]

(cherry picked from commit 8fc0713)

Co-authored-by: EliseevEgor egor.eliseev@jetbrains.com
Co-authored-by: sunmy2019 59365878+sunmy2019@users.noreply.github.com
Co-authored-by: Nikita Sobolev mail@sobolevn.me

• Issue: Return the correct exit code if all tests have been skipped in unittest #106584

[ambv]

This is a bug in 3.12.0, the fix should land for 3.12.1.

[sodul]

@miss-islington, @ambv I suspect this change is breaking green on skipped tests: CleanCut/green#277

Green is tracking test duration by setting a time when startTest is called then calculates the duration in stopTest, unfortunately the changes here are now skipping startTest but not stopTest, if I read the changes correctly.

I will work with the green maintainer to update green to be compatible. There is no issue if no tests are skipped.