ci/lava: Cleanups and debugging for lava_job_submitter
Garbage-collect the TEST_SUITE variable (from @anholt and already reviewed), be better about tracebacks (from @tomeu), as well as adding optional YAML dumping and a validate-only mode (from me).
I don't think we're going to ever get properly stable env with YAML, so I'm going to pick up !11192 (closed) and align it much more closely with bare-metal by using common scripts we can share instead of trying to keep two different things in two different languages in sync. That might take a bit of to-and-fro though and this is useful in the meantime.