Raise an error if the path given to compile_fail matches no files

[leighmcculloch]

The compile_fail function accepts a string to match against paths. If there's a typo in that string which causes it to match no folder / files the test will pass, but it is probably indicative of a developer error.

I experienced this myself when I renamed the folder where I keep trybuild test vectors, without updating the compile_fail call.

The compile_fail function could raise an error in this case.