Add regression test for BRIN parallel builds

Add regression test for BRIN parallel builds

Adds a regression test for parallel CREATE INDEX for BRIN indexes, to
improve coverage for BRIN code, particularly code to allow parallel
index builds introduced by b43757171470.

The test is added to pageinspect, as that allows comparing the index to
one built without parallelism. Another option would be to just build the
index with parallelism and then check it produces correct results. But
checking the index is exactly as if built without parallelism makes
these query checks unnecessary.

Discussion: https://postgr.es/m/1df00a66-db5a-4e66-809a-99b386a06d86%40enterprisedb.com