Fix ClientSurfaceEventsTest.frame_timestamp_increases (#358)

ClientSurfaceEventsTest.frame_timestamp_increases's "wait" logic was
wrong.

We cannot expect strictly monotonic timestamps unless we wait for the
first frame callback: There is no way for the client to know the state
of the server.

(Only if there were no need for a wait could the test be correct)

So, split the test in two:

1. wait for the first frame callback and assert timestamps increases
2. don't wait and assert timestamps do not decrease