chore(workflow): tests: add node version "15.x" to tests

Author: hasezoey

.github/workflows/tests.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: [10.x, 12.x, 14.x]
+        node-version: [10.x, 12.x, 14.x, 15.x] # TODO: change "15.x" to "16.x" when 16 releases
     steps:
       - uses: actions/checkout@v2
         if: github.event.inputs.git-ref == ''