[swift-build-dev] SwiftPM Test Fixtures

[Ryan Lovelett]

If I have a number of `*.json` files that are meant to be loaded and
used as a part of my SwiftPM test suite:

1. how do I get SwiftPM to bundle them with the test bundle?
2. how do I reference the fixtures in the test bundle?

## What I've tried...

I tried creating a top-level directory called `Fixtures`.  Then
referencing the bundle with something like:
`Bundle.allBundles.first(where: { $0.bundlePath.hasSuffix(".xctest")
})!.bundleURL`.

Is this even possible?