Pin major version of actions/checkout action used in test workflow

It's best practices to use release versions of the actions unless there is a compelling reason to do otherwise.

Author: per1234

.github/workflows/test.yml

@@ -22,7 +22,7 @@ jobs:
         run: git config --global core.autocrlf false
 
       - name: Checkout
-        uses: actions/checkout@master
+        uses: actions/checkout@v2
 
       - name: Set Node.js 10.x
         uses: actions/setup-node@v1