Don't use fork to start the notebook in js tests

It can encounter a weird segfault on OS X with sqlite when Qt is present (?!)

The main reason to use the fork was to get the port number,
but this is easy now that notebooks write a server-info file.

Further advantage is that the symptom of a failed server start
is no longer silence and hanging tests, but an actual failure with the server's log output.
html and js tests require sqlite3 (session manager)
don't launch js tests if the server didn't start
halt stream capturer after server terminates
Merge pull request #5339 from minrk/iptest-qt-js-wtf

Don't use fork to start the notebook in js tests