[bootstrap] Add support for running tests.
[bootstrap] Add support for running tests.

 - This should be the last major piece we need to be able to run the package
   manager tests in CI. <rdar://problem/23433999>

mxcl/swift-package-manager,vmanot/swift-package-manager,mxcl/swift-package-manager,vmanot/swift-package-manager,JGiola/swift-package-manager,haskellswift/swift-package-manager,abertelrud/swift-package-manager,haskellswift/swift-package-manager,abertelrud/swift-package-manager,vmanot/swift-package-manager,sschiau/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,vmanot/swift-package-manager,sschiau/swift-package-manager,abertelrud/swift-package-manager,sschiau/swift-package-manager,apple/swift-package-manager,rsmoz/swift-package-manager,vmanot/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,mxcl/swift-package-manager,mxcl/swift-package-manager,apple/swift-package-manager,haskellswift/swift-package-manager,sschiau/swift-package-manager,sschiau/swift-package-manager,rsmoz/swift-package-manager,haskellswift/swift-package-manager,JGiola/swift-package-manager,JGiola/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager