Add alternative output for ON CONFLICT toast isolation test.

On some buildfarm animals the isolationtest added in 07ef0351 failed, as
the order in which processes are run after unlocking is not
guaranteed. Add an alternative output for that.

Discussion: <7969.1471484738@sss.pgh.pa.us>
Backpatch: 9.6, like the test in the aforementioned commit

src/test/isolation/expected/insert-conflict-toast_1.out

@@ -0,0 +1,15 @@
+Parsed test spec with 3 sessions
+
+starting permutation: s2insert s3insert s1commit
+pg_advisory_xact_lock
+
+               
+step s2insert: 
+  INSERT INTO ctoast (key, val) VALUES (1, ctoast_large_val()) ON CONFLICT DO NOTHING;
+ <waiting ...>
+step s3insert: 
+  INSERT INTO ctoast (key, val) VALUES (1, ctoast_large_val()) ON CONFLICT DO NOTHING;
+ <waiting ...>
+step s1commit: COMMIT;
+step s3insert: <... completed>
+step s2insert: <... completed>