Support a "no_server" flag in test config.
* If no_server is true, an instance of rippled should already be running.
* This can be used to debug npm tests in rippled.
* Note that most tests don't clean up after themselves, so developer may
* want to use "mocha --debug <filename.js>" and use JS breakpoints to start and stop rippled manually as needed.
* Also, modify all tests to respect the server_default value.