Fix selectivity estimation @> (anymultirange, anyrange) operator

Fix selectivity estimation @> (anymultirange, anyrange) operator

Attempt to get selectivity estimation for @> (anymultirange, anyrange) operator
caused an error in buildfarm, because this operator was missed in switch()
of calc_hist_selectivity().  Fix that and also make regression tests reliably
check that selectivity estimation for (multi)ranges doesn't fall.  Previously,
whether we test selectivity estimation for (multi)ranges depended on
whether autovacuum managed to gather concurrently to the test.

Reported-by: Michael Paquier
Discussion: https://postgr.es/m/X%2BwmgjRItuvHNBeV%40paquier.xyz