test: add test stage with mongoose v5

Author: nodkz

.github/workflows/nodejs.yml

@@ -23,6 +23,10 @@ jobs:
         run: yarn test
         env:
           CI: true
+      - name: Testing with previous mongoose versions
+        run: yarn test-prev-vers
+        env:
+          CI: true  
       - name: Send codecov.io stats
         if: matrix.node-version == '12.x'
         run: bash <(curl -s https://codecov.io/bash) || echo ''