Updated XmlTestExecutionVisitor to include ITestResultMessage.Output value (+ tests)

This change updates XmlTestExecutionVisitor.CreateTestResultElement to include the value of ITestResultMessage.Output in the xml element returned from the method if Output is not null or whitespace. Additionally, I updated the AddsPassingTestElementToXml and AddsFailingTestElementToXml tests to include output values + added a new test EmptyOutputStringDoesNotShowUpInResultingXml to test the scenario of a ITestResultMessage without an Output value.