Fix scrollWidthHeight-negative-margin-002.html (#37721)

Since we don't support `writing-mode` yet, we got different testcases
with the same title, some passing and some failing. So we had to expect
both PASS and FAIL, making the test useless to detect regressions.

This changes the test to only generate testcases for supported values of
`writing-mode`.

Testing: This is just a test change.

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

tests/wpt/meta/MANIFEST.json

@@ -623925,7 +623925,7 @@
       ]
      ],
      "scrollWidthHeight-negative-margin-002.html": [
-      "1192680c0351cf63259974e2ef46de93158fb83a",
+      "d084f4b396e85f56b9c63d9f028fcae58010a343",
       [
        null,
        {}