[ArrowStringArray] TST: more parameterised testing - part 4
still outstanding:
`test_astype_string` in `pandas/tests/extension/base/casting.py` requires separate PR with fix
`test_convert_dtypes` in `pandas/tests/frame/methods/test_convert_dtypes.py` requires discussion https://github.com/pandas-dev/pandas/pull/40747#issuecomment-812508672... probably need to include in (or follow on to #39908) to at least use the global default
`test_to_html_formatters` in `pandas/tests/io/formats/test_to_html.py`
paramaterisation where pd.StringArray is used directly... could potentially combine with #40962 and expose ArrowStringArray publicly

the changes here to `test_repeat_with_null` need a separate PR to fix so could be excluded here.