adds macos-15-large image option

Author: ChrisC

.github/workflows/voiceover-test.yml

@@ -39,7 +39,8 @@ on:
         options:
           - "13"
           - "14"
-        default: "14"
+          - "15-large"
+        default: "15-large"
       browser:
         description: |
           The browser to use for testing, "chrome" or "firefox" or "safari", default "safari"