TST: Fix shares_memory for arrow string dtype
- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

```bash
mypy.....................................................................Failed
- hook id: mypy
- duration: 131.55s
- exit code: 1

pandas/_testing/__init__.py:1051: error: "ExtensionDtype" has no attribute "storage"  [attr-defined]
pandas/_testing/__init__.py:1058: error: "ExtensionDtype" has no attribute "storage"  [attr-defined]
Found 2 errors in 1 file (checked 1445 source files)
``` thx Thanks @phofl 