Capture average time to load /users/{userId} with cache

[unicornsleuth]

Closes #308

This PR uses Puppeteer to load the /users/2 page and time it. It does this 40 times, taking & saving screenshots, and averages the time.

My results: Total time: 21.789999999999996 seconds Average time: 0.5447499999999998 seconds

To test:

• fork my repo & checkout the branch yaholl/sprint6
• navigate to /test/ui
• in terminal, run node time-users-id.js
• you should see confirmation of 40 screenshots being taken, along with the time it took to load the page each time
• at the end, you will see the Total time and Average time
• you can look in test/results/ui-test-results/users-id
  • you should see a new folder called group- and the current date/time
  • inside, there should be 40 screenshots of the page

[natni]

Great work!