CI: update test runner to use macos-15-intel.

macos-13 is deprecated and will be unavailable after December 8th.

Author: martinwhitaker

.github/workflows/test.yml

@@ -14,7 +14,7 @@ jobs:
   mac:
     strategy:
       fail-fast: false
-    runs-on: macos-13
+    runs-on: macos-15-intel
     name: '🍏 macOS'
     steps: