gh-140657: Run test_import single phase init tests in a subprocess

[vstinner]

Some tests such as test_basic_multiple_interpreters_main_no_reset() leak memory. Run them in a subprocess to avoid leaking memory.

Remove @no_rerun decorator, it's no longer needed.

• Issue: Adding tests to test_import.SubinterpImportTests causes refleak on FreeBSD #140657

[encukou]

Is this better than @no_rerun? It seems that this approach serves the same purpose, and @no_rerun is simpler (and faster).

[vstinner]

I wrote #140712 to add @no_rerun instead.