Update test.yml

Author: toroidal-code

.github/workflows/test.yml

@@ -27,7 +27,7 @@ jobs:
     - name: Install LLVM and Clang
       uses: KyleMayes/install-llvm-action@v1
       with:
-        version: "16"
+        version: "15"
 
     - name: Configure CMake
       run: cmake -Bbuild -DBUILD_TESTS=YES