tests: Fix Qt dependencies.

Partially fixes
<#882>.
This follows commit dae92490.

* tests/CMakeLists.txt: Rename QML_LIBS to QT_LIBS, a regression
introduced in d82e3820.  Remove the
dependency on on the Widgets module, now provided via QT_LIBS.
* tests/qml/main.cpp [WITH_WEBENGINE]: Include QtWebEngine modules
conditionally.
(main) [WITH_WEBENGINE]: Initialize webengine conditionally.

Change-Id: I0df4e11ca3571338fb1e8967784dc6f09ca1dfee