qt_internal_add_test doesn't call qt_import_qml_plugins

Currently tests created by qt_internal_add_test don't have qmlimportscanner run for them (via qt_import_qml_plugins).
This means that in a static Qt build, if the test needs some qml plugins, the test will fail at runtime saying some qml import has failed.

In examples added by the public qt_add_executable, we run qt_import_qml_plugins via scope finalizers.

For tests we have a few options:

• call qt_import_qml_plugins directly in qt_internal_add_test()
• Call it via scope finalizer (only this one function)
• Call all scope finalizers like we do in qt_add_executable. This might prove problematic to implement.

We can't call qt_import_qml_plugins when building the qtdeclarative tests as part of the overall qtdeclarative build (or a top-level build), but we can do it for standalone test builds, which is what is used by the CI.