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 991bb0f965.

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

Per buildfarm member crake.

src/test/modules/worker_spi/worker_spi.c

@@ -173,6 +173,15 @@ worker_spi_main(Datum main_arg)
 		BackgroundWorkerInitializeConnection(worker_spi_database,
 											 worker_spi_role, flags);
 
+	/*
+	 * Disable parallel query for workers started with BYPASS_ALLOWCONN so as
+	 * these don't attempt connections to the database that may not allow
+	 * that.
+	 */
+	if (flags & BGWORKER_BYPASS_ALLOWCONN)
+		SetConfigOption("max_parallel_workers_per_gather", "0",
+						PGC_USERSET, PGC_S_OVERRIDE);
+
 	elog(LOG, "%s initialized with %s.%s",
 		 MyBgworkerEntry->bgw_name, table->schema, table->name);
 	initialize_worker_spi(table);