TST GH26807 Break up test_strings
This PR partially addresses xref #26807 in the case of pandas/tests/test_strings.py. It deletes that file and makes a new subpackage pandas/tests/strings/ with 8 new test modules plus a conftest file. 

I have only moved tests and flattened the structure by moving the tests from being methods to functions -  I haven't changed them in any other way. There are 1814 tests that were in pandas/tests/test_strings.py and are now in pandas/tests/strings/. I tried to keep each test module logically cohesive and less than 1000 lines long - test_strings.py still contains all the tests that don't really fit anywhere else.

- [ ] closes #xxxx
- [ ] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry
