autotests: Reduce execution time of testWaylandSeat

Currently the test spends a lot of time idling. This change speeds up
the test by removing QTest::qWait() calls.

The sync() helper was introduced to sync both client and server sides.
It lets us simplify some code and get rid of QTest::qWait()s.

With this change, the execution time will drop from 20s to about 300ms.