Make unittests fail when bed's tests fail.

This isn't pretty yet. I'd prefer not having the ugly stack bellow my
pretty tests. There're somethings I'm going to leave for the binary
distribution (a packaged test runner) to implement, but this kind of
breaks the point of doing things prettily.
Remove failing tests now that they interfere.

This is just so travis-ci can run. I'll factor out this mock tests as an
example and write actual tests for describe, it and Context.