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.
Why not just have a file, `test/config-debug.js`, that they can copy over?  It means that if you add more to that file, you don't have to change these instructions.

Quibble - you're a little inconsistent about using `code format`.  I'd suggest doing it for each name  that's an actual file or a command:  `rippled`, or `test/config.js`.