Add first draft of a `spin-test` SIP

[rylev]

This SIP describes the spin-test tool that allows running unmodified Spin applications in WebAssembly component based testing environment where all potential imports to the Spin app are virtualized and can be configured by the user in a language independent way.

Implementation of this SIP has already begun here.

[radu-matei]

What is the status of the SIP wrt the actual implementation from https://github.com/fermyon/spin-test?

[rylev]

@radu-matei There haven't been any fundamental objections to the approach and we're far enough along in the implementation that I think this could be merged. I would just need to update the document to reflect the true state of the proposal.