TST: find solution for extension/tests_numpy.py (PandasArray base extension tests)
We currently run the base extension tests also for the `PandasArray` (to have coverage of this array class), but because we actually don't allow to store this in a Series/DataFrame, the tests do some patching to workaround this:

https://github.com/pandas-dev/pandas/blob/dec7d213fb27e9eafb61201f173c241d7105f772/pandas/tests/extension/test_numpy.py#L36-L54

However, this regularly gives problems, see eg https://github.com/pandas-dev/pandas/pull/39991#discussion_r581114674 or https://github.com/pandas-dev/pandas/pull/39722#discussion_r573896611, where for some reason those PandasArrays do / don't get preserved in a DataFrame/Series due to some other code changes that impact this, and the tests are not written to deal with it. 
(one example: `extract_array` will convert a pandas Series with numpy dtype into a PandasArray, when the patch is enabled. This can then cause the result to include the PandasArray EA, while the expected result is not created that way)

So we should try to find a solution for this (as it gets quite annoying to deal with it just to get those tests passing). But I don't directly know what the best solution would be. 
Some ideas:

- Simply remove `tests/extension/test_numpy.py` and thus don't run the base extension interface tests for PandasArray. We still have `tests/arrays/test_numpy.py` for some other tests. The easiest solution, but would also remove test coverage.
- In some way only run the tests in `tests/extension/test_numpy.py` that only deal with the array, and not with the array stored in DataFrame/Series. 

cc @TomAugspurger @jbrockmendel 