pgsql: Backpatch regression tests added by 2d689babe3cb

Backpatch regression tests added by 2d689babe3cb

A new plpgsql test function was added in 14 and up to cover for a bugfix
that was not backpatchable.  We can add it to older versions as a way to
cover other bits of DDL event triggers, with an exception clause to
avoid the problematic corner case.

Originally authored by Michaël Paquier.

Backpatch: 10 through 13.

Discussion: https://postgr.es/m/202205201523.7m5jbfvyanmj@alvherre.pgsql

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ac78cec9811b88e63854406a938f1de6673f3047

Modified Files
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src/test/regress/expected/event_trigger.out | 59 +++++++++++++++++++++++++++--
src/test/regress/sql/event_trigger.sql      | 28 +++++++++++++-
2 files changed, 82 insertions(+), 5 deletions(-)