Paint worklets: Add pref for blocking sleep to be enabled for wpt tests

tests/wpt/mozilla/meta/MANIFEST.json

    "reftest"
   ],
   "mozilla/worklets/test_paint_worklet_timeout.js": [
-   "83cdbd59c905de898b72ba47d3ae10bb95d76301",
+   "3e4ff24fc85ad2956b741ff1785cecfb9641d792",
    "support"
   ],
   "mozilla/worklets/test_paint_worklet_timeout_ref.html": [

tests/wpt/mozilla/meta/mozilla/worklets/__dir__.ini

@@ -1 +1 @@
-prefs: [dom.worklet.enabled:true]
+prefs: [dom.worklet.enabled:true,dom.worklet.blockingsleep.enabled:true]

tests/wpt/mozilla/tests/mozilla/worklets/test_paint_worklet_timeout.js

@@ -1,7 +1,12 @@
 registerPaint("testgreen", class {
     paint(ctx, size) {
+        try {
+            sleep(30); // too long for a paintworklet to init
+        } catch (e) {
+            console.log("Problem sleeping: " + e);
+        }
+        // should fail if control reaches here before timeout
         ctx.fillStyle = 'green';
         ctx.fillRect(0, 0, size.width, size.height);
-        sleep(30); // too long for a paintworklet to init
     }
 });