autotests: Reduce execution time of testLockScreen

testLockScreen uses !eventSpy.wait() expression to check whether the
given signal is emitted. QSignalSpy::wait()'s default timeout is 5s,
which is too much. We don't need to wait that much, in fact, even
wl_display_sync() would suffice. In either case, let's pass explicit
timeout of 10ms which should be good enough.

Before

    Executed in   60.49 secs    fish           external
       usr time    3.92 secs    1.03 millis    3.92 secs
       sys time    2.29 secs    0.16 millis    2.29 secs

After

    Executed in   19.38 secs    fish           external
       usr time    1.64 secs  347.00 micros    1.64 secs
       sys time    1.15 secs   54.00 micros    1.15 secs