TST: base test for ExtensionArray.astype to its own type + copy keyword
- [] closes #28488
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
n.b. I'm not certain. I get no output on the command line when I run this command.
- [x] whatsnew entry
