Run the test workflow in a newer Node version (#4251)

In Node 6, running Karma gave the following warning:

Error during loading "/home/runner/work/backbone/backbone/node_modules/karma-sauce-launcher" plugin:
  Unexpected token function

I had no such error running karma after installing karma-sauce-launcher
locally in Node 14, so this upgrade might fix some things.

We are not exactly in the business of supporting old Node versions, so
this should be relatively painless.

.github/workflows/tests.yml

@@ -24,7 +24,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions/setup-node@v2
         with:
-          node-version: 6
+          node-version: 14
           cache: 'npm'
       - name: Install dependencies
         run: |