Avoid using INFO elevel for what are fundamentally debug messages.

Avoid using INFO elevel for what are fundamentally debug messages.

Commit 6f6b99d13 stuck an INFO message into the fast path for
checking partition constraints, for no very good reason except
that it made it easy for the regression tests to verify that
that path was taken.  Assorted later patches did likewise,
increasing the unsuppressable-chatter level from ALTER TABLE
even more.  This isn't good for the user experience, so let's
drop these messages down to DEBUG1 where they belong.  So as
not to have a loss of test coverage, create a TAP test that
runs the relevant queries with client_min_messages = DEBUG1
and greps for the expected messages.

This testing method is a bit brute-force --- in particular,
it duplicates the execution of a fair amount of the core
create_table and alter_table tests.  We experimented with
other solutions, but running any significant amount of
standard testing with client_min_messages = DEBUG1 seems
to have a lot of output-stability pitfalls, cf commits
bbb96c370 and 5655565c0.  Possibly at some point we'll look
into whether we can reduce the amount of test duplication.

Backpatch into v12, because some of these messages are new
in v12 and we don't really want to ship it that way.

Sergei Kornilov

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