Fix coverage checking - #8057

@sklam thanks for the feedback. WIth testing across Linux and Windows, and different ways of invoking it, e.g.

python run_coverage.py
python ./run_coverage.py
./run_coverage.py
# etc.

I found there's a bunch of fiddly issues still to resolve that seem a bit pointless to plough much time into. Instead, I think it's better to document the workflow using the coverage CLI tool, so I've updated this PR accordingly.

• Do you follow these exact steps?
• Do you have any other coverage-related tips?

Yes, that's how I use it mostly. (referring to the added docs)

The only other tip is to only get a report on changed files when reviewing PRs to speed things up. I use something like:

#! /usr/bin/env ipython
changed_files = !git diff --name-only origin/main...
print("changed files...")
for f in changed_files:
    print("  ", f)
changed_files = ','.join(changed_files)
!coverage xml -o cov.xml --include=$changed_files

Thank you for the patch!