Add test for "flux create source git"

The create source tests are more interesting than the existing tests as they
create objects then wit for the flux source reconciler to complete. The tests
simulate this with a background goroutine that waits for an object to be
created then uses a test specific function to update it.

The tests set a timeout so that if there is a failure they timeout somewhat
quickly rather than hanging for a longer period of time.

Signed-off-by: Allen Porter <allen@thebends.org>