LibJS/test-js: Clean up test-js code

This commit also exposes JSONObject's implementation of stringify to the
public, so that it can be used by test-js without having to go through
the interpreter's environment.

Libraries/LibJS/Tests/test-common.js

@@ -425,7 +425,7 @@ describe = (message, callback) => {
     suiteMessage = defaultSuiteMessage;
 }
 
-test = (message, callback) => {
+const getTestFunction = successMessage => (message, callback) => {
     if (!__TestResults__[suiteMessage])
         __TestResults__[suiteMessage] = {};
 
@@ -437,13 +437,15 @@ test = (message, callback) => {
     try {
         callback();
         suite[message] = {
-            passed: true,
+            result: successMessage,
         };
     } catch (e) {
         suite[message] = {
-            passed: false,
+            result: "fail",
         };
     }
 }
 
+test = getTestFunction("pass");
+
 })();