remove duplication in application runner structure within Dash testing framework

[rpkyle]

We currently repeat ourselves a bit within the the application runners (RRunner and JuliaRunner) -- we should consider 🌴 these up a bit, and call ProcessRunner._start_command instead. We could refactor such that we use self.__class.__name__ within the logger, with a command template string. That would enable us to collapse the logic into a single function.

@alexcjohnson @waralex