Add event loop to test-runner

[jhugman]

According to The Big O of Code Reviews, this is a O(n) change.

This PR adds a crude event loop to the test runner. Specifically:

• setTimeout/clearTimeout/setImmediate. This is largely derived from https://github.com/tmikov/hermes-jsi-demos/blob/master/evloop/evloop.cpp
• an implementation of the ReactCommon::CallInvoker interface
• an asyncTest method to the asserts library

The runner now runs until there are no more tasks left, either from setTimeout or from in-flght callbacks on other threads.

This does not need to be more robust than that: any long-running Rust tasks should protect the test-runner from exiting by setting a timeout in the test.

This is done implicitly by the asyncTest method.