TST: Add test for x-axis labels in subplot with `secondary_y=True`
- [x] closes #14102  (Replace xxxx with the GitHub issue number)
- [x] [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
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

This PR adds a unit test to `pandas/tests/plotting/test_series.py` to verify that X-axis labels are preserved when using `secondary_y=True` and creaitng a new subplot. Adds a test for the solved issue described in https://github.com/pandas-dev/pandas/issues/14102.

Thanks @jasonmokk 