Improved stability of tst_qstatemachine, mark as parallel-safe

Improved stability of tst_qstatemachine, mark as parallel-safe

tst_QStateMachine::postEventFromOtherThread() assumed that 10,000
iterations of the event loop in a separate thread could always be
completed successfully within the default QTRY_COMPARE timeout.
This may be false if the CPU load is already high - such as when running
other tests concurrently.

Decrease the iteration count.

Change-Id: I780523f73c0c16fa0b2ab3201b2b0affdebc198d
Reviewed-by: Toby Tomkins <toby.tomkins@nokia.com>
Reviewed-by: Kalle Lehtonen <kalle.ju.lehtonen@nokia.com>