Restore grandfathered tests removed in #2054

With the move to UTS 35 for language tag processing, the expected
canonicalisation results for "cel-gaulish" should now be consistent across
implementations.

test/intl402/Locale/extensions-grandfathered.js

@@ -22,6 +22,18 @@ features: [Intl.Locale]
 ---*/
 
 const testData = [
+    // Regular grandfathered without modern replacement.
+    {
+        tag: "cel-gaulish",
+        options: {
+            language: "fr",
+            script: "Cyrl",
+            region: "FR",
+            numberingSystem: "latn",
+        },
+        canonical: "fr-Cyrl-FR-u-nu-latn-x-cel-gaulish",
+    },
+
     // Regular grandfathered with modern replacement.
     {
         tag: "art-lojban",

test/intl402/Locale/getters-grandfathered.js

@@ -25,6 +25,13 @@ info: |
 features: [Intl.Locale]
 ---*/
 
+// Regular grandfathered language tag.
+var loc = new Intl.Locale("cel-gaulish");
+assert.sameValue(loc.baseName, "xtg");
+assert.sameValue(loc.language, "xtg");
+assert.sameValue(loc.script, undefined);
+assert.sameValue(loc.region, undefined);
+
 // Regular grandfathered language tag.
 assert.throws(RangeError, () => new Intl.Locale("zh-min"));
 

test/intl402/Locale/likely-subtags-grandfathered.js

@@ -66,6 +66,10 @@ const regularGrandfathered = [
         canonical: "jbo",
         maximized: "jbo-Latn-001",
     },
+    {
+        tag: "cel-gaulish",
+        canonical: "xtg-x-cel-gaulish",
+    },
     {
         tag: "zh-guoyu",
         canonical: "zh",