CI: include tests in coverage
It's hard to tell whether all tests are being executed in CI - can we add not omit them from the coverage report, so this'll be easier?
Where can we observe the coverage difference from this change?

I see that coverage is written to `coverage.xml`, but it's not displayed in our logs it should show up in https://app.codecov.io/gh/pandas-dev/pandas Thanks @MarcoGorelli 