don't start new notebook server for each test *method*

it takes much too long.

This change starts a new notebook server for each test class.

Even that is probably too much, but the module organization makes it troublesome
to start a single notebook server for all services tests.
ensure test_no_kernels runs first

since it expects a clean notebook server.
Merge pull request #6 from minrk/fewer-servers

don't start new notebook server for each test method