WiX: package up the Testing cross-module overlay

[compnerd]

This adds the swift-testing cross-module overlay to the distribution on Windows.

[compnerd]

Tested with a prebuild from https://ci-external.swift.org/job/swift-PR-build-toolchain-windows/5830/artifact/build/artifacts/installer.exe with help from @grynspan

Ran the following snippet:

import Testing
import Foundation

@Test func test() {
  let data = Data()
  Attachment.record(data)
}

[stmontgomery]

Yep, this looks like what you'd need to get the cross-import overlay working. LGTM, thanks!

[grynspan]

@compnerd Does this need to be cherry-picked?

[etcwilde]

Approved with a note, I don't actually see an accepted proposal for cross-import libraries. The Swift-Testing proposal only references the pitch thread. I'm not sure what the rules are here, but I'm pretty sure that accepted features should only be built on accepted features.

[compnerd]

@grynspan yes, we should cherry-pick this.

[grynspan]

@etcwilde Cross-import overlays haven't been approved as a supported feature; our use here is necessary because Swift Testing is part of the toolchain and there is a circular dependency between Foundation and Swift Testing if Swift Testing puts these interfaces directly in the main Testing module. @beccadax and colleagues approved this limited use of the feature because of the constraints we're under.

(I'm on leave and won't respond further right now—please ping Becca, @stmontgomery, or @briancroom if you have further questions.)