validate:launcher: Add `%(config_path)s` in the pipeline desc vars
requested to merge thiblahute/gst-devtools:launcher_add_json_path_to_pipeline_desc_known_vars into master
When defining pipelines_descriptions to run test on in a .json file, you might
need to point to paths in the testsuite directory (for media files URIs
for example), you can now do
"pipeline": "filesrc location="$(config_path)s/../medias/some/file.mkv...