pgsql: Add test for inherited CHECK constraint drop

Add test for inherited CHECK constraint drop

This code is insufficiently covered by tests, so add a few small test
cases to immortalize its behavior before it gets rewritten completely by
the project to catalog NOT NULL constraints.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3da13a6257bc08b1d402c83feb2a35450f988365

Modified Files
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src/test/regress/expected/inherit.out | 109 ++++++++++++++++++++++++++++++++++
src/test/regress/sql/inherit.sql      |  39 ++++++++++++
2 files changed, 148 insertions(+)