TST (string dtype): replace string_storage fixture with explicit storage/na_value keyword arguments for dtype creation
The `string_storage` fixture is currently still parametrized as "python", "pyarrow" and "pyarrow_numpy". In light of the PDEP-14 work, we need to get rid of the "pyarrow_numpy" option (since this is replaced by `storage="pyarrrow", na_value=np.nan`). 

This PR introduces a new fixture to replace the current one for a subset of its use cases, where we currently use it to create the different variants of the string dtype. 

(we might also want to have a direct top-level fixture for those string dtype, but: 1) for those tests, the fixture should be named `dtype` and so easier to keep that defined within the file, 2) we already have a top-level `any_string_dtype` fixture, but that also includes `object` dtype)


xref https://github.com/pandas-dev/pandas/issues/54792
Thanks @jorisvandenbossche 