js/LibJS: Move test functions to pure javascript.

The addition of assert functions to Userland/js
was done before we had load(..) implemented. Now
that it exists, it seems like the right move the
test helper functions to pure javascript instead
of poluting js with random global functions.

Libraries/LibJS/Tests/test-common.js

@@ -0,0 +1,16 @@
+
+function AssertionError(message) {
+    var instance = new Error(message);
+    instance.name = 'AssertionError';
+    Object.setPrototypeOf(instance, Object.getPrototypeOf(this));
+    return instance;
+}
+
+function assert(value) {
+    if (!value)
+        throw new AssertionError("The assertion failed!");
+}
+
+function assertNotReached() {
+    throw new AssertionError("assertNotReached() was reached!");
+}