TST: Remove stacklevel checks for deprecated resample kwargs
- [N/A] closes #xxxx (see https://github.com/pandas-dev/pandas/pull/36629#issuecomment-720757082)
- [x] tests added / passed (tests are failing locally but on unrelated things, think it's a problem somewhere in master)
- [x] passes `black pandas` 
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [N/A] whatsnew entry

This removes `stacklevel` requirements from the deprecation warnings for `resample` and `Grouper` kwargs in an attempt to simplify the warning-handling code (see #36629 for "why"). This only changes `tests.resample.test_deprecated`.

The second commit is optional (it just extracts the deprecated argument checking code out from `Grouper.__new__` to declutter the latter); I can revert it if needed.