Fix mark-and-restore-skipping test case to not be a self-join.
There isn't any good reason for this test to be a self-join rather
than a join between separate tables, except that it saved a couple
of SQL commands for setup. A proposed patch to optimize away
self-joins breaks the test, so adjust it to avoid that happening.
Discussion: https://postgr.es/m/
64486b0b-0404-e39e-322d-
0801154901f3@postgrespro.ru