I don't understand how, but I suspect that the threaded server is
somehow involved in the hangs we see after the tests complete. This
marks it as daemonic so that if it somehow escapes it won't keep the
test process alive.
rather than initializing in main and starting in background, which seemed to cause occasional issues.
Also fix checking for dead notebook, which is no longer a subprocess.
Thomas Kluyver
Min RK
Thomas Kluyver