Support a "no_server" flag in test config.

* Will use a running instance of rippled (possibly in a debugger).
* Modify all tests to respect the server_default value.
* Fail test if new account already exists and has a balance.
* README.md with instructions for advanced test debugging, particularly using no_server.

Tentative reviewers @sublimator, @vinniefalco.
This sorta-kinda encourages trying to run more than one test at a time, per server instance, and that way lies pain. 

Most of the time you'd want the suite to spawn a rippled for you. For  the rare times that you want to debug, typically you use `.only` rather than trying to run a whole bunch of tests and `.skip`ing a bunch.