TestWebKitAPI: Initialize NIXEvent structures in PlatformWebViewNix

Call memset for the respective structures in simulateSpacebarKeyPress(),
simulateRightClick() and simulateMouseMove().

The following tests which were previously failing seem to be OK now:
- test_WebKit2_MouseMoveAfterCrash (SEGFAULT)
- test_WebKit2_ResponsivenessTimerDoesntFireEarly (SEGFAULT)
- test_WebKit2_SpacebarScrolling (Failed)

Reviewed by Thiago Lacerda