[ArrowStringArray] CLN: move and rename test_string_methods
follow-up to #40708

in #40708 the str accessor was enabled and tests changed in `pandas/tests/arrays/string_/test_string.py` and `pandas/tests/strings/test_string_array.py`

This PR colocates the str accessor test in `pandas/tests/arrays/string_/test_string.py` with those in `pandas/tests/strings/test_string_array.py` and renames it `test_string_methods` -> `test_capitalize`