Try running tests on a macOS 15 CI runner

Author: culler

.github/workflows/mac-build.yml

@@ -5,6 +5,7 @@ on:
     - "main"
     - "core-8-branch"
     - "core-8-6-branch"
+    - "sequoia-tests"
     tags:
     - "core-**"
 permissions:
@@ -13,7 +14,7 @@ env:
   ERROR_ON_FAILURES: 1
 jobs:
   xcode:
-    runs-on: macos-14
+    runs-on: macos-15
     defaults:
       run:
         shell: bash