Merge lp://qastaging/~vanvugt/mir/fix-1151645 into lp://qastaging/~mir-team/mir/trunk

MirWaitHandle: Add expect_result() method and some unit tests to fix
LP: #1151645.

expect_result() is required to support asymmetric usage patterns of a
MirWaitHandle like:
    mir_foo(x, A);
    mir_foo(x, B);
    mir_wait_for(mir_foo(x, C));
where mir_foo always reuses the same MirWaitHandle for x.

Previously, mir_wait_for would return as soon as any call to mir_foo finished,
meaning only A or B had completed. But clearly what we want is to wait for C.
That is what expect_result allows us to support as re-usable client functions like
mir_foo are added.

If this looks incomplete, that's because the bigger picture with real-world use case and integration test (mir_surface_{get|set}_type) is over here -->
lp:~vanvugt/mir/surface-types