Extend add_qt_test for qmltest by setting the option QMLTEST
when we detect the config qmltestcase.
We also forwards the GUI option to the tests when detected.
This is a requirement for some QtQuickControls2 tests.
Finally when doing a prefix build, we add the install directory
to the QT_PLUGIN_PATH environment variable.
Change-Id: I3b2ecb494955976e98abbcf3d03925c314336122
Reviewed-by: Qt CMake Build Bot
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
2cc865b943