TST: move custom business day tests to own file (#27085)
- [ ] xref #27085 (chips away at, yes)
- [x] 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

Continuation of https://github.com/pandas-dev/pandas/pull/42284
Moving CustomBusinessDay / CDay tests into own file.