Add injection-point test for new multixact CV usage

Before commit a0e0fb1ba5, multixact.c contained a case in the
multixact-read path where it would loop sleeping 1ms each time until
another multixact-create path completed, which was uncovered by any
tests.  That commit changed the code to rely on a condition variable
instead.  Add a test now, which relies on injection points and "loading"
thereof (because of it being in a critical section), per commit
4b211003ec.

Author: Andrey Borodin <x4mmm@yandex-team.ru>
Reviewed-by: Michaël Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/0925F9A9-4D53-4B27-A87E-3D83A757B0E0@yandex-team.ru

src/backend/access/transam/multixact.c

@@ -88,6 +88,7 @@
 #include "storage/proc.h"
 #include "storage/procarray.h"
 #include "utils/fmgrprotos.h"
+#include "utils/injection_point.h"
 #include "utils/guc_hooks.h"
 #include "utils/memutils.h"
 
@@ -868,6 +869,8 @@ MultiXactIdCreateFromMembers(int nmembers, MultiXactMember *members)
 	 */
 	multi = GetNewMultiXactId(nmembers, &offset);
 
+	INJECTION_POINT("multixact-create-from-members");
+
 	/* Make an XLOG entry describing the new MXID. */
 	xlrec.mid = multi;
 	xlrec.moff = offset;
@@ -1480,6 +1483,8 @@ retry:
 			LWLockRelease(lock);
 			CHECK_FOR_INTERRUPTS();
 
+			INJECTION_POINT("multixact-get-members-cv-sleep");
+
 			ConditionVariableSleep(&MultiXactState->nextoff_cv,
 								   WAIT_EVENT_MULTIXACT_CREATION);
 			slept = true;