bpo-45582: Fix test_embed failure during a PGO build on Windows

[neonene]

PGO-instrumented _testembed.exe and getpath.py often miscalculate the prefix path, when launched with all PYTHON* envvars removed.

The new path calculation uses VPATH, which is defined as "..\\.." in pythoncore.vcxproj. From PGO-instrumented folder, VPATH needs to be "..\\..\\.." to resolve prefix.

https://bugs.python.org/issue45582

[zooba]

I like this change better, but maybe we should get VPATH into sysconfig so that it can be used in the test?

[zooba]

maybe we should get VPATH into sysconfig so that it can be used in the test?

I think adding it as sys._vpath but only on Windows is fine for this. Then sysconfig can add it into its dict if it's there.

[neonene]

Thank you for the nice idea. I've finished the implementation.