Rework new SLRU test with injection points

Rather than the SQL injection_points_load(), this commit changes the
injection point test introduced in 768a9fd553 to rely on the two
macros INJECTION_POINT_LOAD() and INJECTION_POINT_CACHED(), that have
been originally introduced for the sake of this test.

This runs the test as a two-step process: load the injection point, then
run its callback directly from the local cache loaded.  What the test
did originally was also fine, but the point here is to have an example
in core of how to use these new macros.

While on it, fix the header ordering in multixact.c, as pointed out by
Alexander Korotkov.  This was an oversight in 768a9fd553.

Per discussion with Álvaro Herrera.

Author: Michael Paquier
Discussion: https://postgr.es/m/ZsUnJUlSOBNAzwW1@paquier.xyz
Discussion: https://postgr.es/m/CAPpHfduzaBz7KMhwuVOZMTpG=JniPG4aUosXPZCxZydmzq_oEQ@mail.gmail.com

src/backend/access/transam/multixact.c

@@ -88,8 +88,8 @@
 #include "storage/proc.h"
 #include "storage/procarray.h"
 #include "utils/fmgrprotos.h"
-#include "utils/injection_point.h"
 #include "utils/guc_hooks.h"
+#include "utils/injection_point.h"
 #include "utils/memutils.h"
 
 
@@ -855,6 +855,9 @@ MultiXactIdCreateFromMembers(int nmembers, MultiXactMember *members)
 		}
 	}
 
+	/* Load the injection point before entering the critical section */
+	INJECTION_POINT_LOAD("multixact-create-from-members");
+
 	/*
 	 * Assign the MXID and offsets range to use, and make sure there is space
 	 * in the OFFSETs and MEMBERs files.  NB: this routine does
@@ -869,7 +872,7 @@ MultiXactIdCreateFromMembers(int nmembers, MultiXactMember *members)
 	 */
 	multi = GetNewMultiXactId(nmembers, &offset);
 
-	INJECTION_POINT("multixact-create-from-members");
+	INJECTION_POINT_CACHED("multixact-create-from-members");
 
 	/* Make an XLOG entry describing the new MXID. */
 	xlrec.mid = multi;

src/test/modules/test_slru/t/001_multixact.pl

@@ -24,7 +24,7 @@ my ($node, $result);
 $node = PostgreSQL::Test::Cluster->new('mike');
 $node->init;
 $node->append_conf('postgresql.conf',
-	"shared_preload_libraries = 'test_slru'");
+	"shared_preload_libraries = 'test_slru,injection_points'");
 $node->start;
 $node->safe_psql('postgres', q(CREATE EXTENSION injection_points));
 $node->safe_psql('postgres', q(CREATE EXTENSION test_slru));
@@ -64,7 +64,6 @@ $node->safe_psql('postgres',
 $creator->query_until(
 	qr/start/, q{
 	\echo start
-	SELECT injection_points_load('multixact-create-from-members');
 	SELECT test_create_multixact();
 });