Fix extension control path tests

Change expected extension to be installed from amcheck to plpgsql since
not all build farm animals has the contrib module installed.

Author: Matheus Alcantara <mths.dev@pm.me>
Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/E7C7BFFB-8857-48D4-A71F-88B359FADCFD@justatheory.com

src/test/modules/test_extensions/t/001_extension_control_path.pl

@@ -64,14 +64,14 @@ is( $ret2,
 # Ensure that extensions installed on $system is still visible when using with
 # custom extension control path.
 my $ret3 = $node->safe_psql('postgres',
-	"select count(*) > 0 as ok from pg_available_extensions where name = 'amcheck'"
+	"select count(*) > 0 as ok from pg_available_extensions where name = 'plpgsql'"
 );
 is($ret3, "t",
 	"\$system extension is installed correctly on pg_available_extensions");
 
 
 my $ret4 = $node->safe_psql('postgres',
-	"set extension_control_path = ''; select count(*) > 0 as ok from pg_available_extensions where name = 'amcheck'"
+	"set extension_control_path = ''; select count(*) > 0 as ok from pg_available_extensions where name = 'plpgsql'"
 );
 is($ret4, "t",
 	"\$system extension is installed correctly on pg_available_extensions with empty extension_control_path"