pgsql: Fix alter_table.sql test case to test what it claims to.

Fix alter_table.sql test case to test what it claims to.

The stanza "SET STORAGE may need to add a TOAST table" does not
test what it's supposed to, and hasn't done so since we added
the ability to store constant column default values as metadata.
We need to use a non-constant default to get the expected table
rewrite to actually happen.

Fix that, and add the missing checks that would have exposed the
problem to begin with.

Noted while reviewing a patch that made changes in this test case.
Back-patch to v11 where the problem came in.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b158e0b1b1bfd2dfce6d66d67dba3c94449f9cac

Modified Files
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src/test/regress/expected/alter_table.out | 32 ++++++++++++++++++++++---------
src/test/regress/sql/alter_table.sql | 11 +++++++----
2 files changed, 30 insertions(+), 13 deletions(-)