Add auto tests for MultipleMouseClickHelper

Qt WebEngine implements a custom handler for multiple mouse clicks.
These tests are intended to test whether the mouse click events are
properly forwarded to the Chromium's RenderWidgetHost.
Custom mouse click test functions have been also added for the custom
mouse click handler.

Change-Id: Ifdc7d09f5e0f0f76c37e137e5743482bf3fb8abb
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>