Make test order independent

postgresql test if randomly executed then executes "SHOW max_identifier_length". Hence
the need to ignore certain predefined sqls that deal with system calls.

Here is the exact failure I get in master if tests are executed in reverse order for postgresql.

```

NestedThroughAssociationsTest#test_has_many_through_has_one_through_with_has_many_source_reflection_preload [/Users/nsingh/dev/rails_edge/rails/activerecord/test/cases/associations/nested_through_associations_test.rb:189]:
1 instead of 0 queries were executed.
Queries:
SHOW max_identifier_length.
Expected: 0
  Actual: 1
```