Merge lp://qastaging/~afrantzis/mir/fix-or-workaround-1523621 into lp://qastaging/mir

tests: Ensure client surface is in a steady state in tests using a nested setup (LP: #1523621)

tests: Ensure client surface is in a steady state in tests using a nested setup

Due to the many layers of asynchronous processing when using a nested setup, it's difficult to guarantee that the client surface will be in a steady state (visible, focused, ready for input etc) when we need it to. We work around this issue by using an implicit test to check for a steady state: we start sending input events from the device, and wait until we start receiving them on the client side. We have already employed this tactic in test_nested_input.cpp.

Note: Very rarely the new code fails, i.e., we never get input events while waiting for the steady state. This is not related to the test itself, but indicates a race somewhere in our infrastructure. In my testing the failure rate is at least two orders of magnitude lower than the current state of the code, so I think it's worth it over what we have now.