Slow and very slow tests have now their own test category. We add SlowTest
for tests which are slower than ~5ms and VerySlowTest if they are slower
than ~100ms. They are disabled them by "-*SlowTest.*". If you have a faster
machine than most developers simply try lower values. The aim is that most
developers can execute the tests in under ~2s.

In the long run we should use dependency breaking and data sharing to
reduce the count of the slow tests.

Change-Id: I8578071258d7f89b2052709f3dd526ced811483f
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>