leave out last test and add first CI file

.github/workflows/cmake-multi-platform.yml

+name: multi platform test
+
+on:
+  push:
+    branches: [ "trunk", "develop" ]
+  pull_request:
+    branches: [ "trunk" ]
+
+jobs:
+  build:
+    runs-on: ${{ matrix.os }}
+
+    strategy:
+      fail-fast: false
+
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+        include:
+          - os: windows-latest
+            cpp_compiler: cl
+          - os: ubuntu-latest
+            cpp_compiler: g++
+          - os: macos-latest
+            cpp_compiler: clang++
+
+    steps:
+    - uses: actions/checkout@v3
+
+    - name: Set reusable strings
+      id: strings
+      shell: bash
+      run: echo "build-output-dir=${{ github.workspace }}/build" >> "$GITHUB_OUTPUT"
+
+    - name: Install Catch2 Ubuntu
+      if: matrix.os == 'ubuntu-latest'
+      run: >
+        sudo apt-get update && sudo apt-get install catch2
+
+    - name: Install Catch2 macOS
+      if: matrix.os == 'macos-latest'
+      run: >
+        brew install catch2
+
+    - name: Configure CMake
+      run: >
+        cmake -B ${{ steps.strings.outputs.build-output-dir }}
+        -DCMAKE_CXX_COMPILER=${{ matrix.cpp_compiler }}
+        -DCMAKE_BUILD_TYPE=Release
+        -S ${{ github.workspace }}
+        
+    - name: Build
+      run: cmake --build ${{ steps.strings.outputs.build-output-dir }} --config Release
+
+    - name: Test
+      working-directory: ${{ steps.strings.outputs.build-output-dir }}
+      run: ctest --build-config Release --output-on-failure
+