chore: Add test for synthesized image documents with CSP headers. (#38191)

Adds a regression test for https://github.com/servo/servo/pull/38186,
verifying that the CSP header on the image is ignored when it's loaded
as a document.

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

tests/wpt/meta/MANIFEST.json

@@ -39126,6 +39126,23 @@
      ]
     ]
    },
+   "content-security-policy": {
+    "generic": {
+     "image-document-ignores-csp.html": [
+      "a3f6f64854d98d40c8c405831d6f3f9656fdf34d",
+      [
+       null,
+       [
+        [
+         "/content-security-policy/generic/image-document-ignores-csp-ref.html",
+         "=="
+        ]
+       ],
+       {}
+      ]
+     ]
+    }
+   },
    "contenteditable": {
     "synthetic-height.html": [
      "4d466bff412e6de924b9886ef808b49f7ff2b44d",
@@ -401739,6 +401756,10 @@
       "5c580273dcfc94eff137a0ae65314bebc9b7b5c0",
       []
      ],
+     "image-document-ignores-csp-ref.html": [
+      "e0747fd6c2810ccd771c5e33795d8c46ba56ff6c",
+      []
+     ],
      "negativeTests.js": [
       "44b4d7f683d8fb674a3c2b5d22eb8ea9d7b31ada",
       []
@@ -402755,6 +402776,14 @@
       "2fa1e0ac0663a65deae6602621521cc2844b93de",
       []
      ],
+     "pass3.png": [
+      "2fa1e0ac0663a65deae6602621521cc2844b93de",
+      []
+     ],
+     "pass3.png.headers": [
+      "6581fd425ed9da6b6273d1eefb84ea9ac9ac6e53",
+      []
+     ],
      "ping.js": [
       "750ae45f969491ea29eaa47c85f96f97d94b415f",
       []