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.

Reviewers @sublimator, @rec. Optional @wltsmrz.
How about I modify the `config-example.js` to do everything to set up the debug server? Then the instructions only require that first sub-step (`exports.server_default`).  There is precedent for this: https://wiki.ripple.com/Visual_Studio_2013_Build_Instructions#Create_a_test_config.js

I'll go ahead and clean up the filenames and commands.