amcheck: Harden tests against concurrent autovacuums.

amcheck: Harden tests against concurrent autovacuums.

The previous coding of the test was vulnerable against autovacuum
triggering work on one of the tables in check_btree.sql.

For the purpose of the test it's entirely sufficient to check for
locks taken by the current process, so add an appropriate restriction.
While touching the test, expand it to also check for locks on the
underlying relations, rather than just the indexes.

Reported-By: Tom Lane
Discussion: https://postgr.es/m/30354.1489434301@sss.pgh.pa.us