Allow Rex::Test to run against remote machines

Currently Rex::Test is strongly tied to creating a VM to connect to, execute some task(s) against the VM, and then do a series of basic checks. It would be nice to lift that restriction and make Rex::Test able to connect to any remote Rex is capable to handle (perhaps including local connections too).

This would enable users to run the very same tests against their "other" machines, like test/stage/production environment, to validate/inspect their current state.

This could also pave the way to make Rex::Tests itself pluggable, making it easier for users to choose their preferred testing flavors/suites, including, but not limited to:

• sticking to the basic built-in tests
• use Rex::Test::Spec to use an RSpec-like DSL (cc @chenryn)
• use Rex::Test::Sparrow which was proposed during a chat with @melezhik via IRC

Start by tracing the Rex::Test flow that creates a VM, connects to it, runs tasks, and performs the basic checks. Define how that entry point could accept local or remote machines supported by Rex, then verify that the existing checks work without requiring VM creation.