WiX: Add _TestingInterop

[jerryjrchen]

This includes the library created in swiftlang/swift-testing#1369

Will move out of draft once I build the toolchain and confirm it works as expected.

Resolves rdar://163673609

[jerryjrchen]

I'm admittedly just copying similar examples already here, so I wanted to call this part out. The .lib in my Windows toolchain seems to be static libs:

jerryjrchen@JJC-WINDOWS:/mnt/c/Users/jerryjrchen/AppData/Local/Programs/Swift/Platforms/0.0.0$ file ./Windows.platform/Developer/Library/Testing-0.0.0/usr/lib/swift/windows/aarch64/Testing.lib
./Windows.platform/Developer/Library/Testing-0.0.0/usr/lib/swift/windows/aarch64/Testing.lib: current ar archive

I assume this means we support building both statically and dynamically linked test bundles (binaries?) on Windows?

[grynspan]

Windows exposes library content differently than you're used to. Let's sync up later today and I'll try to share what little I know.

[compnerd]

The changes look correct. Please do a cross-repo toolchain build before merging.

The lib files are import libraries (conceptually like a tbd) and the DLL are the runtime bits (conceptually like a dylib).

[jerryjrchen]

Confirmed the libraries are in the right locations with a toolchain install:

jerryjrchen@JJC-WINDOWS:/mnt/c/Users/jerryjrchen/AppData/Local/Programs/Swift/Platforms/0.0.0/Windows.platform/Developer/Library/Testing-0.0.0/usr$ find . | grep Interop
./bin32/_TestingInterop.dll
./bin64/_TestingInterop.dll
./bin64a/_TestingInterop.dll
./lib/swift/windows/aarch64/_TestingInterop.lib
./lib/swift/windows/i686/_TestingInterop.lib
./lib/swift/windows/x86_64/_TestingInterop.lib