Add ASV Benchmarks for GroupBy Ops on Datetimelike

[WillAyd]

xref comment in #19673, we don't have any ASV benchmarks in groupby.GroupByMethods to measure performance on datetimelike types (timestamp / timedelta)

[WillAyd]

xref #19722 - as part of this it also makes sense to clean up some of the existing GroupBy tests and get rid of any duplication