CI: Consolidate unit test rules, codecov pathfix

[marshallward]

This commit contains two changes to gcov report generation and codecov upload.

1. Separation of the file parser unit tests from the others was causing them to be exluded from report.cov.unit.

   This patch reworks the rules to replace MOM_file_parser_tests.F90.gcov with driver code, build/unit/test_%.F90.gcov.

   This does assume at all drivers will look the same (test_*.F90) but that part can be reworked if it ever becomes a problem in the future.

   Thanks to @adcroft for multiple suggestions in this PR.

2. Github appears to internally store all its repositories in another directory with the name as the repo (in this case MOM6/).

   Normally this is hidden to everyone, but it was causing some confusion with the codecov upload tool, and was unable to match the source code to the .gcov report.

   The .codecov.yml config file was modified to adjust for this path change, and should now correctly allow coverage to be reported alongside the file.

   (The GitHub Actions app likely makes this adjustment, but we need to do it manually since we upload directly to Codecov.io.)

[adcroft]

This should be high priority since it fixes the lack of codecov reports and cleans up a mess I left in the .testing Makefile. I'd at least like to see this accepted before #741 since that PR will be well served by a proper codecov report.

[adcroft]

OK, since #741 is up next, I'm testing this one at https://gitlab.gfdl.noaa.gov/ogrp/mom6ci/MOM6/-/pipelines/25386