API tests: Make them compile with QtWebEngine.

They still won't link since most methods aren't implemented yet.

The changes to tst_qwebengineframe.cpp are mostly esthetic as those
tests should be merged with tst_qwebenginepage.cpp once the
implementation has progressed a bit further.

Change-Id: If6627820069b51d7461d667716aafa6e250a684b
Reviewed-by: Andras Becsi <andras.becsi@digia.com>