Include code that runs in runpytest_subprocess in coverage

[bluetech]

In #7651 we had some code which is only covered by a subprocess test and is somewhat hard to cover inprocess.

It seems that the code running in the subprocess is not covered. @nicoddemus mentioned that some coverage configuration is needed to get this to work: https://coverage.readthedocs.io/en/stable/subprocess.html

[asottile]

one way I've done this before is via coverage-enable-subprocess

essentially:

1. install coverage-enable-subprocess under test
2. https://github.com/asottile/setuptools-golang/blob/a6eaed0801255d4b2b7fb9e964126bce2747a0a9/tox.ini#L6
3. https://github.com/asottile/setuptools-golang/blob/a6eaed0801255d4b2b7fb9e964126bce2747a0a9/.coveragerc#L5

[antonblr]

Closing via #8188 - lines which have been shown as uncovered in #7651 are covered now.