Xcode_16 will soon require macos-15 in GHA

Author: paulb777

.github/workflows/abtesting.yml

@@ -24,8 +24,13 @@ jobs:
       matrix:
         # TODO: macos tests are blocked by https://github.com/erikdoe/ocmock/pull/532
         target: [ios, tvos, macos --skip-tests, watchos]
-        os: [macos-14]
-        xcode: [Xcode_15.2, Xcode_16]
+        include:
+          - os: macos-13
+            xcode: Xcode_15.2
+          - os: macos-14
+            xcode: Xcode_15.4
+          - os: macos-15
+            xcode: Xcode_16
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v4