Run pipeline for a custom apptests branch

To test apptests branch run a pipeline on sai-marvell-api repo with a pipeline variable APPTESTS_REF set to your apptests branch name.
The variable also accepts commits and tags.
NOTE: you probably don't have permissions to run a pipeline on sai-marvell-api master, so choose your own sai-marvell-api branch or branch off of master and push with no additional commits.