CI: Enable testing on macOS

Author: alexdewar

.github/workflows/cargo-build-and-test.yml

@@ -18,7 +18,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [windows-latest, ubuntu-latest]
+        os: [windows-latest, macos-latest, ubuntu-latest]
         toolchain:
           - stable
     steps: