Switch partitions when running in serve mode

An idea from @runspired: When running tests with `ember exam --split=n --partition=1 --serve`, or something similar, it would be nice to be able to switch partitions without having to quit and restart or manually modify parameters.

Currently, Testem offers support for `q` to quit and `enter` to rerun tests, but maybe it can expose an interface to extend those controls, such that Ember Exam can offer partition switching?