Add tap test for pg_signal_autovacuum role

This commit provides testig coverage for ccd38024bc, checking that a
role granted pg_signal_autovacuum_worker is able to stop a vacuum
worker.

An injection point with a wait is placed at the beginning of autovacuum
worker startup to make sure that a worker is still alive when sending
and processing the signal sent.

Author: Anthony Leung, Michael Paquier, Kirill Reshke
Reviewed-by: Andrey Borodin, Nathan Bossart
Discussion: https://postgr.es/m/CALdSSPiQPuuQpOkF7x0g2QkA5eE-3xXt7hiJFvShV1bHKDvf8w@mail.gmail.com

src/backend/postmaster/autovacuum.c

@@ -100,6 +100,7 @@
 #include "utils/fmgroids.h"
 #include "utils/fmgrprotos.h"
 #include "utils/guc_hooks.h"
+#include "utils/injection_point.h"
 #include "utils/lsyscache.h"
 #include "utils/memutils.h"
 #include "utils/ps_status.h"
@@ -1902,6 +1903,12 @@ do_autovacuum(void)
 	/* Start a transaction so our commands have one to play into. */
 	StartTransactionCommand();
 
+	/*
+	 * This injection point is put in a transaction block to work with a wait
+	 * that uses a condition variable.
+	 */
+	INJECTION_POINT("autovacuum-worker-start");
+
 	/*
 	 * Compute the multixact age for which freezing is urgent.  This is
 	 * normally autovacuum_multixact_freeze_max_age, but may be less if we are