pgsql: Adjust alter_table regression test to avoid conflicts with

Log Message:
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Adjust alter_table regression test to avoid conflicts with rangefuncs
and prepare tests, which cause intermittent failures in parallel test
mode. Back-port of fix originally applied to 8.0 and 7.4 branches;
the problems do not appear to exist in 7.2 branch but they do occur
in 7.3. Per buildfarm results.

Tags:
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REL7_3_STABLE

Modified Files:
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pgsql/src/test/regress/expected:
alter_table.out (r1.53.2.1 -> r1.53.2.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/alter_table.out.diff?r1=1.53.2.1&r2=1.53.2.2)
pgsql/src/test/regress/sql:
alter_table.sql (r1.34.2.1 -> r1.34.2.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/alter_table.sql.diff?r1=1.34.2.1&r2=1.34.2.2)