Add emit/test (#3933)

Summary:
Note that two tests are passing on internal but not OSS. Skip for now.

Change the test backend name from BackendWithCompilerDemo to BackendWithCompilerExample because BackendWithCompilerDemo is packaged by exir/backend/test/test_backends.py and could cause confusion in pytest.

Pull Request resolved: https://github.com/pytorch/executorch/pull/3933

Reviewed By: JacobSzwejbka

Differential Revision: D58421581

Pulled By: kirklandsign

fbshipit-source-id: f89edcba323ee8d6c49e885c11acaf05877b7496