[ArrayManager] Remaining GroupBy tests (fix count, pass on libreduction for now)
Follow-up on #39885 and #40047

This passes / skips all groupby tests (in `/tests/groupby`). Actual code changes:

- fix `groupby().count()` to not create a ArrayManager with 2D arrays
- for now skip the usage of `fast_apply` / `libreduction.apply_frame_axis0` for DataFrames with ArrayManager. This can be tackled separately, while for now already getting more test coverage by using the python fallback (as we eg also do for EAs).
- implement `ignore_failures` for `ArrayManager.grouped_reduction` (this keyword is actually always True at the moment, so we could also leave away the keyword itself)

Most of the skips are related `quantile` (or `describe`) not yet being implemented. And it also uncovered a few other failures, that can be worked on as follow-ups.
