Change test tables in copy2 regression test to be temporary tables.

This prevents autovacuum from reclaiming free space in them and causing
the test's output row order to change, which is causing intermittent
bogus failure reports in the buildfarm.

Backpatch to 8.3.  The issue exists further back, but since autovacuum was
not on by default before 8.3, it's not a problem for buildfarm testing.