Add a regression test for allow_system_table_mods
Add a regression test file that exercises the kinds of commands that
allow_system_table_mods allows.
This is put in the "unsafe_tests" suite, so it won't accidentally
create a mess if someone runs the normal regression tests against an
instance that they care about.
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/
8b00ea5e-28a7-88ba-e848-
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