add a test for invalid timer removal

We cover such cases; yet its better to have a test.

src/browser/tests/window/timers.html

@@ -23,7 +23,6 @@
   });
 </script>
 
-
 <script id=setTimeout>
   testing.expectEqual(1, window.setTimeout.length);
   let wst2 = 1;
@@ -32,3 +31,11 @@
   }, 1, 2, 3);
   testing.eventually(() => testing.expectEqual(5, wst2));
 </script>
+
+<script id=invalid-timer-clear>
+  // Surprisingly, these don't fail but silently ignored.
+  clearTimeout(-1);
+  clearInterval(-2);
+  clearImmediate(-3);
+  testing.expectEqual(true, true);
+</script>