create #clearErrors in MockValidator to help in tests

I have some tests where a String should be not empty (!= null and size > 0).

Nowadays I need to do this:
<pre>
		String cepWithoutDash = "05594130";
		address.setCep(cepWithoutDash);
		address.validate(validator);
		assertTrue(validator.containsMessage("validation.realEstate.invalidCep"));
		
		validator = new MockValidator();
		String cepWithMoreNumbersAfterDash = "05594-1301";
		address.setCep(cepWithMoreNumbersAfterDash);
		address.validate(validator);
		assertTrue(validator.containsMessage("validation.realEstate.invalidCep"));
</pre>

I need to create a new MockValidator to clear errors from first validation. With `clearErrors` method this tests can be easily created.