Make src/test/recovery/t/017_shm.pl safe for concurrent execution.
Make src/test/recovery/t/017_shm.pl safe for concurrent execution.

Buildfarm members idiacanthus and komodoensis, which share a host, both
executed this test in the same second.  That failed.  Back-patch to 9.6,
where the test first appeared.

Discussion: 0fa1f35bd1f7a8da93fa9c7fd1b9e190ab2bcd9b@rfd.leadboat.com

xinzweb/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb