Migrate test-demo-backend-delegation to a separate job (#1078)

Summary:
Instead of using test-models and test.sh, we use a separate job, because this job is quite different, and we want to get rid of the hack from the additional arg.

See https://github.com/pytorch/executorch/actions/runs/6631141293/job/18014013866?pr=1078 for logs

Pull Request resolved: https://github.com/pytorch/executorch/pull/1078

Reviewed By: huydhn

Differential Revision: D50611594

Pulled By: kirklandsign

fbshipit-source-id: abcc03dec57484a7c6fb8629214d978d7a6b9067