[ping] Add a real ping test. (#125)
[ping] Add a real ping test. (#125)

* This test will run only if ENABLE_EXTERNAL_TESTS environment variable is set.

* Since we are adding a ping test, and apparently github actions doesn't support [ping](https://github.com/actions/virtual-environments/issues/1519#issuecomment-683790054), we're not adding these tests to Github Actions yet.


cloudprober/cloudprober,cloudprober/cloudprober,cloudprober/cloudprober