swift-package-manager

[bootstrap] Add support for building tests on OS X.

 - We build the tests into bundles, so that they are compatible with the system XCTest.

 - Not yet enabled by default, this is run with --build-tests.

 - No support yet for running the built tests.

 - This only applies to the bootstrap build -- the package manager itself does
   not yet have support for building test bundles.