Add system matrix for testing workflow

Author: jerray

.github/workflows/main.yml

@@ -5,7 +5,10 @@ on:
 jobs:
   Test:
     name: Testing
-    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macOS-latest]
+    runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout
         uses: actions/checkout@v1