Fix: Added tests that check comparisonClause() (do not merge)

AS requested by tractorcow: https://github.com/silverstripe/silverstripe-postgresql/pull/26

Tested on MySQL (succeeds as expected)  Tested on PostgreSQL (and fails as expected, but passes with patch). The fixes test only DataQueryTest related items for consistency, e.g. by avoiding DataObject calls.
Column names should be quoted as per ansi standard. These tests should allow columns of any case.