Bootstrap a testing harness target.

[grynspan]

This PR adds a (stub, unimplemented, incomplete, etc.) harness executable target that uses Swift Argument Parser. This target will eventually act as a host for tests, but for now just throws an error if invoked.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[finagolfin]

Oh nice, how would this work for cross-compilation: this new harness runs on the host and communicates to the current test runners on a separate target platform over the network, or this harness runs on the target platform itself? Maybe you don't intend it for cross-compilation altogether, just seeing how we can use it.

[grynspan]

@finagolfin Short answer: it should probably be possible to implement cross-device testing for Android with this tool, though we'll need your help to do it.

The exact feature set is currently unspecified; this PR just creates the new target. My initial focus will be on making it work on-device, but it will be leveraging our JSON event stream and there's no reason it couldn't draw events from off-device sources given the right application of elbow grease.

[stmontgomery]

@finagolfin I plan to post a proposal on the forums fairly soon describing the anticipated longer-term trajectory here, this is just a starting point as @grynspan mentioned. It's definitely something I hope can eventually support running tests on remotely-connected devices though, and it's great to hear your interest in that.