TST: [stats] add tests for the moments

Mark known fails as such.
BUG: stop liming the support for betaprime distribution.

Cutting off the support at a large value does not seem necessary.
All tests pass, no new problems seem to show up.
MAINT: deduplicate some tests (discrete vs continuous distributions)

Move tests common to discrete and continuous distributions to a new
common_tests.py, and import from there.
MAINT: address comments
MAINT: typos, fixed
Merge pull request #2857 from EvgeniBurovski/moments_

TST: [stats] add tests for the moments