Tests for expected errors

[sampsyo]

When testing compilers, it's often really useful to write tests that are intended to trigger an error in the compiler—for example, to assert that some invalid code is correctly rejected by a type checker.

It seems like we need two things for this:

1. Nonzero exit status codes. We currently check that every test exits with status 0; it should be possible to override this and check for any exit code. The easy way would be with a comment. A different, somewhat silly idea would be to have a separate file just for containing the exit code, which would preserve the "diff to see changes" workflow that we have now.
2. Capture stderr (optionally?). If an error message is written to stderr instead of stdout, we should be able to make that a checkable output file, just like stdout and literal files are now.

[avanhatt]

+1 we may end up doing this for our bril testing, will PR if so!

[sampsyo]

We implemented both of these long ago!