add node v16 test env

Author: kazupon

.github/workflows/test.yml

@@ -24,7 +24,7 @@ jobs:
         os: [ubuntu-latest, windows-latest, macos-latest]
         # node v10 disable, see https://github.com/jsdom/jsdom/issues/2795
         # node: [10, 12, 14]
-        node: [12, 14]
+        node: [12, 14, 16]
     steps:
       - name: Checkout
         uses: actions/checkout@v2