pgsql: Fix an ALTER TABLE test case so that it actually tests what the

Log Message:
-----------
Fix an ALTER TABLE test case so that it actually tests what the comment says it
is testing.  Ah, the perils of making keywords optional ...

Modified Files:
--------------
    pgsql/src/test/regress/sql:
        alter_table.sql (r1.58 -> r1.59)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/alter_table.sql?r1=1.58&r2=1.59)
    pgsql/src/test/regress/expected:
        alter_table.out (r1.107 -> r1.108)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/alter_table.out?r1=1.107&r2=1.108)