[MASSMAIL]pgsql: Don't use the pg_am system catalog in new test

Don't use the pg_am system catalog in new test

This causes deadlocks because it's a highly trafficked catalog.  Use a
regular table created by the same test instead.

Discussion: https://postgr.es/m/f3e61e27-19d0-5e40-3eb2-53282fa0532a@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/13b3b62746ec8bd9c8e3f0bc23862f1172996333

Modified Files
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src/test/regress/expected/create_am.out | 4 ++--
src/test/regress/sql/create_am.sql      | 2 +-
2 files changed, 3 insertions(+), 3 deletions(-)