[WIP] test(e2e): added jest env and puppeteer for e2e

[anikethsaha]

E2E testing using puppeteer and jest

• added puppeteer
• removed mocha
• added jest
• created custom jest env for supporting the live-server instance in tests suites
• npm script support
• eslint fixes ( Eslint fixes for the project #987 )
• test cases using puppeteer
  • setup image snapshot using jest-image-snapshot
  • sideBar testing
  • search field testing
  • cover page
• fix jest config to support running other tests in test folder
• travis additions
• coverage (nyc or jest and codecoverage for PRs)
• Github action if required

Currently, This PR contains a fixture which is a copy of our docs. So these tests will be run against the updated docs with updated themes and plugins.
Refactored the e2e/fixtures/docs/index.html in order to use packages from local project

This PR doesnt contain SSR tests

More tests and fixtures can be added in later PR and in future.

Update/Note

Currently I have assigned failureThreshold to 10% but in future I think we should consider <1%

Test timing is currently 30sec , may consider to increase it if required max 60sec