Handle exit messages from tests in ex_unit

Fixes #982

Spawns a new process where we run a single test. This way we can trap_exit for a single test and add it as a failure if it emits exit. Is this an okay solution?

Doesn't run `teardown` at the moment. Is it okay run `teardown` from a different process? If so, I will run `teardown` when I receive the exit message.

There are no tests for the failure handling in exunit atm. How can we do that? Creating a custom formatter?