Use stricter ordering in regression test query for pg_stat_io

The query introduced in 8b532771a0 is proving to have ordering issues
under at least the locale cs_CZ.  This commit updates the query to use a
stricter ordering.

Per reports from buildfarm members hippopotamus and jay.

src/test/regress/expected/stats.out

@@ -14,7 +14,7 @@ SHOW track_counts;  -- must be on
 -- List of backend types, contexts and objects tracked in pg_stat_io.
 \a
 SELECT backend_type, object, context FROM pg_stat_io
-  ORDER BY backend_type, object, context COLLATE "C";
+  ORDER BY backend_type COLLATE "C", object COLLATE "C", context COLLATE "C";
 backend_type|object|context
 autovacuum launcher|relation|bulkread
 autovacuum launcher|relation|init

src/test/regress/sql/stats.sql

@@ -11,7 +11,7 @@ SHOW track_counts;  -- must be on
 -- List of backend types, contexts and objects tracked in pg_stat_io.
 \a
 SELECT backend_type, object, context FROM pg_stat_io
-  ORDER BY backend_type, object, context COLLATE "C";
+  ORDER BY backend_type COLLATE "C", object COLLATE "C", context COLLATE "C";
 \a
 
 -- ensure that both seqscan and indexscan plans are allowed