Don't run fast_default regression test in parallel with other tests.

Don't run fast_default regression test in parallel with other tests.

Since it sets up an event trigger that would fire on DDL done by any
concurrent test script, the original scheduling is just an invitation
to irreproducible test failures.  (The fact that we found a bug through
exactly such irreproducible test failures doesn't really change the
calculus here: this script is a hazard to anything that runs in parallel
with it today or might be added to that parallel group in future.  No,
I don't believe that the trigger is protecting itself sufficiently to
avoid all possible trouble.)

Discussion: https://postgr.es/m/5767.1523995174@sss.pgh.pa.us