Add explicit VACUUM to stabilize plans in inherit tests

Add explicit VACUUM to stabilize plans in inherit tests

On stock PostgreSQL, CREATE INDEX also updates statistics in pg_class
(relpages and reltuples). But Postgres-XL does not do that, which may
result in plan differences when the test relies on this behavior.

This is the same issue as in cfb055553687c257dd1d1ed123356c892f48a804,
but affecting inherit regression tests. So fix it in the same way, by
doing an explicit vacuum on the tables.