worker_spi: Bump up max_worker_processes in TAP tests

worker_spi: Bump up max_worker_processes in TAP tests

mamba has detected a failure in the last test that should start a
bgworker while bypassing the role login check.  The buildfarm did not
provide any information about its failure in the logs, but I suspect
that this is caused by an exhaustion of the max_worker_processes slots
set at 8 by default.

In "normal" test runs, the number of bgworkers running at this stage of
the test is already 7, so, if one of them spawns for example a parallel
worker all the slots would be taken, preventing the last worker of the
test to start.

Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/ZSyebsiub88pyJJO@paquier.xyz