validate:runner: Add a method to force exiting the runner

validate:runner: Add a method to force exiting the runner

This method is similar to runner_printf() but can be used
only once. The user needs to make sure all the pipeline
are in NULL state when this is called.

The method emits a "STOPPING" signal and at that point
overrides or monitors should do extra processing/checks if
needed.

+ Make use of it everywhere where it makes sense.

API:
  gst_validate_runner_exit
  GstValidateRunner::stopping signal