test_launcher fails if a test py.ini is left behind

[encukou]

PreservePyIni in test_launcher.py writes a temporary file to %LOCALAPPDATA%\py.ini, and restores/deletes it when done.
If the restore fails (or, presumably, sometimes when multiple tests run at the same time), the test config is left and subsequent tests will fail.

I reproduced locally. Here's a buildbot run showing the same symptom: https://buildbot.python.org/#/builders/1295/builds/174/steps/4/logs/stdio

Would it make sense to use a file in the test directory, and add a PYLAUNCHER_CONFIG_FILE env variable with the filename?
Or should this use some system-wide lock?

[zooba]

Adding the variable to override the config file makes sense. Can't promise I'll get to implementing it immediately, but the design works for me.

[eryksun]

If the restore fails

That means a preexisting "%LocalAppData%\py.ini" would be lost. Barring the proposed change, this testing scheme could try to be more conservative. PreservePyIni could rename an existing "py.ini" to a numbered backup of the form "py.ini.nnnn". Then try to remove "py.ini" and restore the backup. If restoring the backup were to fail, at least it could still be restored manually.