fix: add missing testTarget to Package.swift

The tests directory exists with ReadmeVerificationTests.swift but Package.swift
was missing the testTarget declaration, causing 'no tests found' error in CI.

Also updated CI workflow to pass --traits URLRouting flag to swift test commands,
as FormCoding tests require URLRouting trait to access dependency modules.

Author: coenttb

Package.swift

@@ -41,6 +41,10 @@ let package = Package(
                 ),
                 .product(name: "MultipartFormCoding", package: "swift-multipart-form-coding"),
             ]
+        ),
+        .testTarget(
+            name: "FormCoding Tests",
+            dependencies: ["FormCoding"]
         )
     ]
 )