TST: misplaced string array test
Just encountered this because I wanted to add a test there for another PR. 
Those tests were moved and splitted in https://github.com/pandas-dev/pandas/pull/46136, but the "string" tests are not actually about StringArray, they are about PandasArray with string numpy dtype (`np.dtype("U")`), and thus belong with the others in `tests/arrays/numpy_`, since `tests/arrays/string_` is for StringArray
thanks @jorisvandenbossche 