Add support for Testing module

[marcprux]

Macros work with the latest swift 6 Android toolchain on a macOS host (🎉), but Testing is not included in the built SDK, resulting in errors when building a newly created skip package init project:

[2/12] Emitting module swift6_test_demoTests
/Users/marc/Desktop/swift6-test-demo/Tests/swift6-test-demoTests/swift6_test_demoTests.swift:1:8: error: no such module 'Testing'
1 | import Testing
  |        `- error: no such module 'Testing'
2 | @testable import swift6_test_demo
3 | 
[3/12] Compiling swift6_test_demoTests swift6_test_demoTests.swift
/Users/marc/Desktop/swift6-test-demo/Tests/swift6-test-demoTests/swift6_test_demoTests.swift:1:8: error: no such module 'Testing'
1 | import Testing
  |        `- error: no such module 'Testing'
2 | @testable import swift6_test_demo
3 | 

I had added --swift-testing --install-swift-testing to the build-script in #167 and added swift-testing to get-packages-and-swift-source.swift, hoping that would be enough to build and include the module, but that doesn't seem sufficient.

Is there some additional step that is needed to get the Testing.swiftmodule built and included?