layout: Add testing API for counting restyled and rebuilt fragments (#42817)

Now `ServoTestUtils.forceLayout()` will provide the number of fragments
that have been restyled and rebuilt. This will be useful to test that
incremental layout works well.

Testing: Adds a test using this new API

---------

Signed-off-by: Oriol Brufau <obrufau@igalia.com>
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Luke Warlow <lwarlow@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

tests/wpt/mozilla/meta/MANIFEST.json

@@ -13032,6 +13032,13 @@
     ]
    },
    "incremental-layout": {
+    "basic-fragment-tree-layout.html": [
+     "f249b241dd97d20295efd923d77a12fac873a911",
+     [
+      null,
+      {}
+     ]
+    ],
     "layout-phases-display-list.html": [
      "0f08ae12a4610987d304bdd2431ccde0bec192be",
      [