Fix hardcoded paths in utils.js

[rajcoss]

Currently, the server has hardcoded paths in utils.js

https://github.com/ciena-blueplanet/webdriverio-server/blob/master/testUtils/utils.js#L148

      screenshotRoot: 'tests/e2e/screenshots',
      failedComparisonsRoot: 'tests/e2e/screenshots/diff',

This assumes that the tests and screenshots are located in tests/e2e and tests/e2e/screenshots folder. We should populate this programmatically using input from test-config.js.