Disable slow tree log within `ember test --server`.

The `testem` UI does not always repaint the full screen, which means
that in some scenarios the slow tree log bleeds through and there is no
testem command to for repaint the full screen (that I know of at least).
Merge pull request #877 from rjackson/silence-watcher-in-test-server

Disable slow tree log within `ember test --server`.