Added tests that check comparisonClause()

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.
Merge pull request #3509 from tractorcow/pr/3364

Added tests that check comparisonClause()