Cleanup tst_QWebEngineScript::injectionPoint

The code was not correctly giving a function as argument to the
addEventHandler, and the whole logic was more complicated than it
needed to be.

Change-Id: I21e4e6bb6adf1071ae7eb2798d129224af8ef0fb
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>