Don't test PostgreSQL implementation details

Asserting that the message contains "QUERY PLAN" is enough for the adapter's test.
The plan may vary according to number of tuples etc. but that's out of our concern.

activerecord/test/cases/adapters/postgresql/explain_test.rb

@@ -11,16 +11,13 @@ def test_explain_for_one_query
           explain = Developer.where(:id => 1).explain
           assert_match %(EXPLAIN for: SELECT "developers".* FROM "developers" WHERE "developers"."id" = $1), explain
           assert_match %(QUERY PLAN), explain
-          assert_match %(Index Scan using developers_pkey on developers), explain
         end
 
         def test_explain_with_eager_loading
           explain = Developer.where(:id => 1).includes(:audit_logs).explain
           assert_match %(QUERY PLAN), explain
           assert_match %(EXPLAIN for: SELECT "developers".* FROM "developers" WHERE "developers"."id" = $1), explain
-          assert_match %(Index Scan using developers_pkey on developers), explain
           assert_match %(EXPLAIN for: SELECT "audit_logs".* FROM "audit_logs" WHERE "audit_logs"."developer_id" IN (1)), explain
-          assert_match %(Seq Scan on audit_logs), explain
         end
       end
     end