layout: Include spanned gutters when laying out contents of table cell (#38290)

If a cell would e.g. span 2 columns, each 50px wide, separated by a 10px
gutter, then we used to lay out the contents of the cell with a 100px
wide containing block. Now we will include the size of the gutter.

Testing: Adding new test
Fixes: #38277

Signed-off-by: Oriol Brufau <obrufau@igalia.com>

tests/wpt/meta/MANIFEST.json

@@ -107621,6 +107621,19 @@
         {}
        ]
       ],
+      "border-spacing-095.html": [
+       "8e5c8f55ccaeb749894a63970ac349cfbce98d53",
+       [
+        null,
+        [
+         [
+          "/css/reference/ref-filled-green-100px-square.xht",
+          "=="
+         ]
+        ],
+        {}
+       ]
+      ],
       "border-spacing-applies-to-016.xht": [
        "0536763244237b77645937fe30e4898f1efcbfb5",
        [