Test 39.18 - Insulate error and output regexes from each other.

Because the output is alphabetically sorted, "error:" always comes
before "GRES_PARSABLE." Thus, when looking for "WARNING:" instead,
that comes *after* GRES_PARSABLE and can royally mess things up.

The fix is to insulate the output and error regexes from each other by
running them in separate expect loops. This is way more robust and
will guarantee that they never again interfere with each other.

Bug 7517