worker_spi: Fix test failure with BGWORKER_BYPASS_ALLOWCONN

worker_spi: Fix test failure with BGWORKER_BYPASS_ALLOWCONN

A bgworker can spawn parallel workers of its own when executing queries,
and if the worker uses BGWORKER_BYPASS_ALLOWCONN while the database it
is connected to does not allow connections, a parallel worker would fail
to startup.  In the case of this module, the step checking for the
presence of the schema to create was spawning a worker, failing the last
test introduced by 991bb0f9653c.

This issue could be reproduced with debug_parallel_query = 'regress',
for example.

Per buildfarm member crake.