Python: Add a ShutdownForTest() in mojo.embedder.

Make the Python bindings tests not call mojo::embedder::Init() multiple
times (without shutting down, something that's only supported for
tests). Enable a DCHECK.

(I tried being more clever, e.g., trying to put it into
mojo.test_embedder, but failed.)

R=qsr@chromium.org
BUG=432670

Review URL: https://codereview.chromium.org/721993002