Introduce testing with mocha, proxyquire, sinon and nyc

- mocha is the unit testing framework
- proxyrequire allows module dependencies to be replaced
- sinon creates mocks (fakes used here)
- nyc measures coverage

Only graphql.js is tested, for a simple start.

This adds a lot of dev dependencies, so use `npm ci --production` in
the validate workflow.

Author: foolip

.github/workflows/lint.yml

@@ -1,7 +1,7 @@
 on: [push, pull_request]
 name: lint
 jobs:
-  validate:
+  lint:
     runs-on: ubuntu-18.04
     steps:
     - name: Checkout

.github/workflows/test.yml

@@ -0,0 +1,18 @@
+on: [push, pull_request]
+name: test
+jobs:
+  test:
+    runs-on: ubuntu-18.04
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v1
+    - name: Setup Node.js
+      uses: actions/setup-node@v1
+      with:
+        node-version: 12.x
+    - name: Setup dependencies
+      run: |
+        echo '{}' > config.json
+        npm ci
+    - name: Run tests
+      run: npm test

.github/workflows/validate.yml

@@ -18,7 +18,7 @@ jobs:
     - name: Setup dependencies
       run: |
         echo '{"w3capikey":"${{ secrets.W3C_API_KEY }}","ghToken":"${{ secrets.W3C_GITHUB_TOKEN }}"}' > config.json
-        npm ci
+        npm ci --production
     - name: Run validate.js
       run: node validate.js > report.json
     - name: Push report.json updates

.gitignore

@@ -1,2 +1,3 @@
 config.json
 node_modules/
+.nyc_output/