CI: Try to speed up ArrayManager
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Don't re-run ArrayManager tests if we are already in ArrayManager mode.

P.S. I'm not sure what the arraymanager marker is supposed to be for. If it only runs the frame tests, wouldn't it be better to just specify that directly in the ``PYTEST_AM_CMD``.
