Enable the use of geoclue in every Qt for Linux build

Currently the geoclue plug-in is not shipped as part of the Qt package. It is desirable to do that.

To do that we need to ensure that the auto tests run continue to run while the geoclue plugin is present (auto tests require the test backend). Sometimes the tests accidentally pickup the geoclue backend and tests start failing. The backend must be explicitly set to the test version.

Furthermore it needs to be investigated what happens if geoclue is not installed on a target machine and the user starts QtPositioning example which would load the geoclue plug-in. The most likely oucome would be a runtime error due to the missing link dependency. This situation must be gracefully caught or even prevented.