JS: Update TaintTracking test

javascript/ql/test/library-tests/TaintTracking/stringification-read-steps.js

@@ -0,0 +1,31 @@
+import 'dummy';
+
+function makeObject() {
+    return {
+        foo: {
+            bar: {
+                baz: source()
+            }
+        }
+    };
+}
+
+function test() {
+    const object = makeObject();
+
+    sink(object); // OK
+    sink(JSON.stringify(object)); // NOT OK
+    sink(object); // OK
+}
+
+function testCapture() {
+    const object = makeObject();
+
+    sink(object); // OK
+    sink(JSON.stringify(object)); // NOT OK
+    sink(object); // OK - use-use flow should not see the effects of the implicit read in JSON.stringify
+
+    function capture() {
+        object;
+    }
+}