pgsql: Add a regression test for allow_system_table_mods

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-e848-21528b632354%402ndquadrant.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/7fc380f83d466b43a8f65bb52c925c1ab19736ea

Modified Files
--------------
src/test/modules/unsafe_tests/Makefile             |   2 +-
.../unsafe_tests/expected/alter_system_table.out   | 168 +++++++++++++++++++
.../unsafe_tests/sql/alter_system_table.sql        | 185 +++++++++++++++++++++
src/test/regress/expected/alter_table.out          |   4 -
src/test/regress/sql/alter_table.sql               |   4 -
5 files changed, 354 insertions(+), 9 deletions(-)