Fix the clipboard and network unit tests.

They call mojo::embedder::Init() multiple times (without shutting down).
Note that using the horrible shell_test_helper.* is mutually exclusive
with using //mojo/edk/test:run_all_unittests (since both want to call
Init()).

The Python bindings tests are still broken.

R=jamesr@chromium.org
BUG=432670

Review URL: https://codereview.chromium.org/721723003