Normalising testing strategy

plugins/postcss-double-position-gradients/.github/workflows/test.yml

@@ -7,12 +7,12 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node: [12, 16]
+        node: [12, 14, 16]
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-node@v2
         with:
           node-version: ${{ matrix.node }}
 
-      - run: yarn install --ignore-scripts
-      - run: yarn run test
+      - run: npm install --ignore-scripts
+      - run: npm run test