If one of these asserts fails, then this test will timeout (taking 2
seconds to do so).

By using a small helper (`timeoutPromise`) to implement the "next tick"
functionality that was previously achieved with `setTimeout`, this is
improved: we return a promise from the test function and a failure will
be reported immediately.

See https://mochajs.org/#working-with-promises for more on returning
promises from mocha test functions.