CLN: de-duplicate numeric type check in _libs/testing
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

In `testing.pyx` we keep a list of numeric dtypes and use that for type checking. But we have functions that do this in `tslibs/util.pxd` so it's better to use those instead