swift-package-manager

[Commands] Fix `swift-test` to always build using the correct manifest.

 - It will now always regenerate the build manifest (including cloning, etc, if
   necessary). This also means it now accepts the -Xcc (etc.) arguments that
   swift-build takes. Unfortunately that means they *must* be passed, if they
   were used before, but allowing an inconsistent build was a poor tradeoff for
   that feature.

 - The implementation is fairly gross, as we don't yet have the infrastructure
   we need to share all the common code between swift-build and swift-test (in
   particular to share the command line flags).

 - Resolves SR-1135.