tests: Fix test override execution

[EricCousineau-TRI]

Hotfix for #2353

Ran into this when trying to add a quick test on upstream.

[EricCousineau-TRI]

Er, from looking closer, seems like current master should work. Perhaps I did something wrong. Will try a few more times.

[EricCousineau-TRI]

K, updated (via force push).

Two things:

• The replacement output to PYBIND11_BINARY_TEST_FILES, but then code below consumed PYBIND11_BINARY_PYTEST_FILES (missing PY)
• The replacement basically tells pytest to read the Python files from the binary dir, but it should've been the src dir.

[bstaletic]

I guess @henryiii should have the final word, but this makes sense.

[henryiii]

Looks good to me! Thanks!

[EricCousineau-TRI]

Thanks!