Improve boilerplate for test code

[ianfixes]

Seems like I should have a macro to replace

int main(int argc, char *argv[]) {
  return Test::run_and_report(argc, argv);
}

But I don't know what it should be, or if I should do it at all

Pros:

• Gives Ruby full control over the executables, enables tighter coupling and more seamless transfer of settings there without intervention on the part of the Arduino library maintainer.

Cons:

• What should the macro be, such that when you look at a test file you know it's a "normal" CPP file?