Fix border collapsing at the end of a table-row-group

This fixes the border-end calculation for table rows whose borders are
collapsed with rows in different rowgroups.  The border collapsing code now
uses an iterator that yields all the rows as a flat sequence, regardless of
how they are grouped in rowgroups.  It gets rid of
`TableRowGroupFlow::preliminary_collapsed_borders` which was never correct.
(It was read but never written.)

This may fix #8120 but I'm not 100% certain. (I haven't managed to reproduce
the intermittent failure locally, and my reduced test case still fails but in
a different way.)

tests/wpt/mozilla/meta/MANIFEST.json

@@ -740,6 +740,18 @@
             "url": "/_mozilla/css/border_collapse_missing_cell_a.html"
           }
         ],
+        "css/border_collapse_rowgroup_a.html": [
+          {
+            "path": "css/border_collapse_rowgroup_a.html",
+            "references": [
+              [
+                "/_mozilla/css/border_collapse_rowgroup_ref.html",
+                "=="
+              ]
+            ],
+            "url": "/_mozilla/css/border_collapse_rowgroup_a.html"
+          }
+        ],
         "css/border_collapse_simple_a.html": [
           {
             "path": "css/border_collapse_simple_a.html",
@@ -6910,6 +6922,18 @@
           "url": "/_mozilla/css/border_collapse_missing_cell_a.html"
         }
       ],
+      "css/border_collapse_rowgroup_a.html": [
+        {
+          "path": "css/border_collapse_rowgroup_a.html",
+          "references": [
+            [
+              "/_mozilla/css/border_collapse_rowgroup_ref.html",
+              "=="
+            ]
+          ],
+          "url": "/_mozilla/css/border_collapse_rowgroup_a.html"
+        }
+      ],
       "css/border_collapse_simple_a.html": [
         {
           "path": "css/border_collapse_simple_a.html",

tests/wpt/mozilla/meta/css/border_collapse_simple_a.html.ini

@@ -1,3 +0,0 @@
-[border_collapse_simple_a.html]
-  type: reftest
-  disabled: https://github.com/servo/servo/issues/8120