Provide TestClient.load_job()

[guhcampos]

I recently stumbled into the project and evaluated it for the testing needs of a kubernetes operator I'm working on, but noticed the absence of a TestClient.load_job() feature which I believe makes sense as part of the plugin.

My particular use case, for context, is:

1. load several jobs into the cluster: a failed, a succeeded and a currently running (sleeps for a long time) jobs from a pytest fixture
2. yield
3. run tests which are supposed to react to different job states
4. delete the loaded jobs from the cluster