Rename xctest executable to avoid inferring it as module name

Author: aciidgh

Sources/PackageType/Product.swift

@@ -34,7 +34,7 @@ public class Product {
                 return "lib\(name).so"
             #endif
         case .Test:
-            let base = "\(name).xctest"
+            let base = "\(name)Test.xctest"
             #if os(OSX)
                 return "\(base)/Contents/MacOS/\(name)"
             #else