TDML Runner needs to restrict text used for error comparison

Currently the TDML runner's comparison feature for errors uses the entire backtrace information. So if your test is looking for an error that contains "lengthKind", but your test filename is "lengthKind.tdml", it will match and the test will erroneously pass. As Mike mentioned in our e-mail chain, the TDML runner "should remove all the file-name things, leaving only the actual error message content."