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