pgsql: Add explicit regression tests for ALTER TABLE lock levels.

Log Message:
-----------
Add explicit regression tests for ALTER TABLE lock levels.
Use this to catch a couple of lock level assignments that slipped
through manual testing, per Peter Eisentraut.

Modified Files:
--------------
pgsql/src/backend/commands:
cluster.c (r1.204 -> r1.205)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/cluster.c?r1=1.204&r2=1.205)
tablecmds.c (r1.335 -> r1.336)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c?r1=1.335&r2=1.336)
pgsql/src/include/commands:
cluster.h (r1.41 -> r1.42)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/cluster.h?r1=1.41&r2=1.42)
pgsql/src/test/regress/expected:
alter_table.out (r1.119 -> r1.120)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/alter_table.out?r1=1.119&r2=1.120)
pgsql/src/test/regress/sql:
alter_table.sql (r1.62 -> r1.63)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/alter_table.sql?r1=1.62&r2=1.63)