TST: [ArrowStringArray] add dtype parameterisation to test_astype_float and test_fillna_args
#35259 added dtype parameterisation to existing tests in pandas/tests/arrays/string_/test_string.py and xfailed for ArrowStringArray where needed.

test_astype_float and test_fillna_args were later added in #37974 and #37987 respectively.

This PR doesn't fix (although will be looking at the all the xfailed tests in this module shortly, https://github.com/pandas-dev/pandas/issues/35169#issuecomment-727217462) just xfails to reduce the diff (slightly) for #39908