Allow using the FTL SDK from module integration tests

[alecthomas]

We've previously discussed creating some test helpers for FTL, but haven't because of concerns around maintaining these helpers across all languages. A slightly simpler idea just occurred to me, that we could implement a single helper function that starts FTL and allows the SDK in each language to work within the test. That is, from the test you can use sdk.Call() (and Context.call() in Kotlin), and any other FTL systems. This would keep the per-language code for this relatively self contained, and reuse all of the existing SDK work.

[alecthomas]

@matt2e 's change (design doc) will help facilitate this

[alecthomas]

This is done, effectively.