Switch the macOS CI runs to the macos-15-intel build agent

refs actions/runner-images#13046, because the macOS 13 agents are going away shortly

Some of the unit tests require Mono, and that is no longer installed on the newer macOS build agents, so install it manually for now

Author: Numpsy

.github/workflows/build_and_test.yml

@@ -19,7 +19,7 @@ jobs:
         runs-on: ${{ matrix.os }}
         strategy:
             matrix:
-                os: [ubuntu-22.04, windows-latest, macOS-13]
+                os: [ubuntu-22.04, windows-latest, macos-15-intel]
         steps:
             - uses: actions/checkout@v4
             - name: Setup .NET 6
@@ -28,6 +28,9 @@ jobs:
                   dotnet-version: '6.0.301'
             - name: Restore dotnet tools
               run: dotnet tool restore
+            - name: Install mono (Mac)
+              if: runner.os == 'macOS'
+              run: brew install mono
             - name: remove current fake runner tool
               run: dotnet tool uninstall fake-cli
             - name: Build fake runner