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>
		address.setCep(null);
		address.validate(validator);
		assertTrue(validator.containsMessage("validation.required", i18n("realEstate.cep")));
		
		validator = new MockValidator();
		address.setCep("");
		address.validate(validator);
		assertTrue(validator.containsMessage("validation.required", i18n("realEstate.cep")));
</pre>

I need to create a new MockValidator to clear errors from first validation. With `clearErrors` method this tests can be easily created.