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.
I had assumed that the user would have ready the basic usage on the wiki, which includes the installation step. I went ahead and added a link to the appropriate section here, and added a note to the top indicating that assumption.