Failing fixtures with *.sim files with syntax errors may be misleading

#24 illustrated a problem in hand writing sim files. If the fixture should fail and you test it by running against your own compiler, the test harness will "pass" but for the wrong reason (syntax error and not an error in the phase you're testing). Ideally, there'd be some way to catch these kind of mistakes as a linter (run as part of the test suite testing the harness).