fbp-spec-mocha: Support proxying to a runtime

[jonnor]

Split out of #36.

To multiplex legacy cases and non-legacy cases across the same runtime connection.
It could be that the best approach is to let this mocha-legacy-compat tool proxy communication to the real runtime. Especially since this is a feature we want to have/build also for MsgFlo (msgflo/msgflo#28).

We also need to avoid double-detection of the proper fbp-spec cases when those are using the Mocha runner integration. Should be doable by that integraiton detecting when we are in this legacy mode.