LibJS: Provide 'details' key in results object for duplicate test

The test-js program expects this to exist for 'result: "fail"' results
and would crash if any duplicated test(message) occurs, as we didn't
provide 'details' in that case.

Userland/Libraries/LibJS/Tests/test-common.js

@@ -429,6 +429,7 @@ class ExpectationError extends Error {
         if (suite[message]) {
             suite[message] = {
                 result: "fail",
+                details: "Another test with the same message did already run",
             };
             return;
         }