- Add channel test helpers to k8s.io/client-go/util/testing
- Use the new channel test helpers in the client and server watch
  tests to validate that the result channels closes without error
  when the client stops the watcher.
- Use the new channel test helpers in the client watcher decoder
  tests to validate that encoded watch events can be decoded and
  that the decoder errors with EOF when stopped asynchronously.

These new tests uncovered existing errors (added TODOs):
1.  The watch client doesn't close the response body when it
    encounters a NegotiateError.
2.  The watch server has a race condition that sometimes sends a
    watch error on the result channel after the client watcher has
    been stopped.

- Fix a bug in client-go Watch and WatchList that was keeping the
  response body open when a NegotiateError was encountered by
  Request.newStreamWatcher. This was causing the server to keep the
  storage watcher and timeout channel open until server-side timeout.
- Fix a bug in client-go Watch and WatchList that was sometimes sending
  a "http: read on closed response body" error from the decoder to the
  result channel after the client watcher had been closed, which closes
  the http response body. The watcher is suppossed to be closed by the
  client when done reading from the result channel, so the impact was
  minimal, but this helps avoid needing to drain the result channel
  before closing it.