LibJS: Reorganize tests into subfolders

Author: https://github.com/mattco98
Commit: 4c48c9d69d
Pull-request: https://github.com/SerenityOS/serenity/pull/2681
Reviewed-by: https://github.com/linusg

Libraries/LibJS/Tests/builtins/Date/Date.now.js

@@ -0,0 +1,15 @@
+load("test-common.js");
+
+try {
+    var last = 0;
+    for (var i = 0; i < 100; ++i) {
+        var now = Date.now();
+        assert(!isNaN(now))
+        assert(now > 1580000000000);
+        assert(now >= last);
+        last = now;
+    }
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}