This is out first CallHandler test(!) Switches react-sdk to use
createCall on the client object so we can stub this out in the test.
Add a bunch more stubs to the test client.
There's more stuff in this test that has scope to be used more
widely, like waiting for a certain dispatch and mocking out rooms
with particular sets of users in them: we could consider moving these
out to test utils if we wanted.
dc3d05bc88