Add a expect.fail()

[ulrikstrid]

This is useful when pattern matching and only 1 of the branches are okay. Currently we have to do some dummy-expect that we can make sure fails. Example where it would be useful:

switch (response.body) {
| `String(body) => expect.string(body).toEqual("test")
| _ => expect.fail() /* currently: expect.string("fail").toEqual("me") */
}

[kyldvs]

I would like this too! I would probably make it so that expect.fail() also accepts a string for a description to use in the error message