pgsql: Replace alter_table.sql test usage of event triggers.

Replace alter_table.sql test usage of event triggers.

The test in 93765bd956b added an event trigger to ensure that the
tested table rewrites do not get optimized away (as happened in the
past). But doing so would require running the tests in isolation, as
otherwise the trigger might also fire in concurrent sessions, causing
test failures there.

Reported-By: Tom Lane
Discussion: https://postgr.es/m/3328.1570740683@sss.pgh.pa.us
Backpatch: 12, just as 93765bd956b

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3b25de62099cf954ea0ca1fe5d70dbc3fbb776c5

Modified Files
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src/test/regress/expected/alter_table.out | 105 +++++++++++++++++++++---------
src/test/regress/sql/alter_table.sql | 71 ++++++++++++--------
2 files changed, 116 insertions(+), 60 deletions(-)