Codecov: remove test_utils from `Files`

[ndmanvar]

test file test_utils.py is showing up. might be a good idea to add it to the ignore list. https://docs.codecov.com/docs/ignoring-paths

i just checked the file. the test_utils.py is purely a test file, just assertions and tests, so in theory it shouldn't be counted towards the overall coverage of the repo or "production code". i'm not sure why some tools include the tests as part of the coverage report, but it can be mitigated by excluding them via codecov.yaml or --source setting in the pytest, but the issue is that the test directory is also under the /src/ so it looks like it is instrumenting the code for all the python files in that location. this is not a big deal, but in case anyone asks this is why it is showing up.