Allow tests full access to host WASI implementation

[rylev]

Because of reasons¹, tests also use the virtualized version of WASI that the router and Spin app use. However, we don't want tests to have the same limitations as those other subcomponents. The WASI virtualization already has partial support for passing its implementation on to imports that the host can provide. We need to expand this as well as develop a mechanism that detects whether a call to WASI is in the router/app or in the test itself. When in the router/app, we only allow virtualization, and when in the test, we pass through to the host implementation.

Footnotes

1. This is a well know limitation of virtualization in WASI 0.2 brought on by the fact that async and streams are not first class citizens and thus must be modeled as normal wit interfaces and shared across all other interfaces that require some sort of async or streaming. Thus since we want to virtualization some usage of WASI, we need to virtualize all uses of WASI. ↩