gh-91338: Ensure test_ucnhash_capi_reset doesn't pass PYTHONHOME to _testembed.exe

[neonene]

A minor tweak for Windows. _test_embed.exe fails to import *.pyd modules when the initialization is repeated with PYTHONHOME from python.bat.

>python.bat -m test test_embed -v
======================================================================
FAIL: test_ucnhash_capi_reset (test.test_embed.EmbeddingTests.test_ucnhash_capi_reset)
----------------------------------------------------------------------
Traceback (most recent call last):
File "C:\cpython\Lib\test\test_embed.py", line 349, in test_ucnhash_capi_reset
out, err = self.run_embedded_interpreter("test_repeated_init_exec", code)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\cpython\Lib\test\test_embed.py", line 117, in run_embedded_interpreter

stdout:
9
stderr:
--- Loop #1 ---
--- Loop #2 ---
File "<string>", line 1
print('\N{digit nine}')
^
SyntaxError: (unicode error) \N escapes not supported (can't load unicodedata module)

sys.path at Loop#1 has the build directory, which is replaced with f"{PYTHONHOME}/DLLs" at the next loop. The behavior looks realistically designed.

GitHub CI machines doesn't show the error, using rt.bat where PYTHONHOME is not set.

https://bugs.python.org/issue47182

[tiran]

Looks good to me!

[neonene]

@zooba, could you take a look at this?
I'll close if unnecessary.

[zooba]

We have a few other tests that use the same function. I'd assume we either need to update all of them (because they're all "wrong", even if they're not triggering it yet), or make sure that the values aren't regenerated differently.

Do we know the root cause of the changed value?

[neonene]

Holding the value of platstdlib_dir for getpath.py would fix the root cause.
My implementation was modifying PyConfig and _PyPathConfig structures.
This PR (updated) is less risky for me.

[neonene]

Do we know the root cause of the changed value?

I'll close this and open an issue, considering that the current behavior, which I think is practical, may be unintentional.