Add Gaia endurance tests (v1-train)

This pull adds the endurance testing functionality to gaiatest in addition to pulling in the latest changes made by the functional UI tests in https://github.com/mozilla/gaia-ui-tests.

Due to changes in the JavaScript 'atoms' it was suggested by @lightsofapollo that we merge these subsequent to landing the endurance tests to avoid any unexpected regressions of the JavaScript performance tests.

There will be a subsequent pull request for the master branch.