[SwiftTest] Add a mode to list test from test modules.
[SwiftTest] Add a mode to list test from test modules.

To list the tests we get the JSON representation of the test cases using
the appropriate tool and show the tests in format `ModuleName.TestCase/TestMethod`.
On OSX we use the swiftpm-xctest-helper tool bundled with swift package manager and
the `-dump-tests-json` mode of XCTest on Linux.

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