pgsql: Fix rare deadlock failure in create_am regression test.

Fix rare deadlock failure in create_am regression test.

The "DROP ACCESS METHOD gist2" test will require locking the index
to be dropped and then its table; while most ordinary operations
lock a table first then its index.  While no concurrent test scripts
should be touching fast_emp4000, autovacuum might chance to be
processing that table when the DROP runs, resulting in a deadlock
failure.  This is pretty rare but we see it in the buildfarm from
time to time.

To fix, acquire a lock on fast_emp4000 before issuing the DROP.

Since the point of the exercise is mostly to prevent buildfarm
failures, back-patch to 9.6 where this test was introduced.

Discussion: https://postgr.es/m/839004.1599185607@sss.pgh.pa.us

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/43b6f3ed8b135d8452e820e6489d90bd86c4ef4e

Modified Files
--------------
src/test/regress/expected/create_am.out | 5 +++++
src/test/regress/sql/create_am.sql      | 5 +++++
2 files changed, 10 insertions(+)