Tweak TAP test for worker terminations in worker_spi

Tweak TAP test for worker terminations in worker_spi

The test has been reported as having a race condition for the case of a
worker that should be terminated after a database rename.  Based on the
report received from buildfarm member jay, the database renamed is
accessed by a different session, preventing the ALTER DATABASE to
complete, ultimately failing the test.

Honestly, I am not completely sure what is the origin of this
disturbance, but two possibilities are an autovacuum or parallel worker
(due to debug_parallel_query being used by the host).  In order to
(hopefully) stabilize the test, autovacuum and debug_parallel_query are
now disabled in the configuration of the node used in the test.

The failure is hard to reproduce, so it will take a few weeks to make
sure that the test has become stable.  Let's see where it goes.

Reported-by: Aya Iwata <iwata.aya@fujitsu.com>
Discussion: https://postgr.es/m/OS3PR01MB8889505E2F3E443CCA4BD72EEA45A@OS3PR01MB8889.jpnprd01.prod.outlook.com