For get and filter queries of NamedUserGroup, realm_for_sharding
field is used instead of realm field, as directly using
realm_for_sharding field on NamedUserGroup makes the query faster
than using realm present on the base UserGroup table.
Tests were failing due to credit usage being capture in the next
month instead of current if the current time is last day of the
month. To fix this, we mock current time to not be the last day
of any month.
