build: run darwin tests on macos 10.15, 11 and 12

Signed-off-by: Paulo Gomes <paulo.gomes@weave.works>

.github/workflows/tests.yaml

@@ -58,10 +58,13 @@ jobs:
           GO_TEST_ARGS: ''
         run: make test
 
-  # Runs 'make test' on macos-10.15 to assure development environment for 
+  # Runs 'make test' on MacOS to ensure the continuous support for contributors
-  # contributors using MacOS.
+  # using it as a development environment.
   darwin-amd64:
-    runs-on: macos-10.15
+    strategy:
+      matrix:
+        os: [macos-10.15, macos-11, macos-12]
+    runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout
         uses: actions/checkout@v3