Revert "Remove missing test target declaration"

This reverts commit 5de4dd4.

Author: j-f1

Package.swift

@@ -107,5 +107,9 @@ let package = Package(
             name: "WebIDLToSwift",
             dependencies: ["WebIDL"]
         ),
+        .testTarget(
+            name: "WebAPIKitTests",
+            dependencies: ["DOM"]
+        ),
     ]
 )

Sources/WebIDLToSwift/PackageManifest.swift

@@ -58,6 +58,10 @@ func generateManifest(_ modules: [Module]) -> String {
                 name: "WebIDLToSwift",
                 dependencies: ["WebIDL"]
             ),
+            .testTarget(
+                name: "WebAPIKitTests",
+                dependencies: ["DOM"]
+            ),
         ]
     )
     """#