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.
I used to write things like this, addressed to the reader:  you do this, you do that.

But I've been taken to task for it - because it sounds less professional.  And I felt they were right, so now I write things as a recipe:  do this, do that, type this.  (It also is a little more compact...)