fix(tests): Fix the oauth_webchannel tests when run by themselves.

In beforeEach, FunctionalHelpers.clearBrowserState was being called before any page was loaded, meaning localStorage and sessionStorage were not being cleared for the content server. This has the side effect that the tests contained a subtle bug where they unknowingly depended on other tests to sign a user in.

The fix is to load the content server's page before running FunctionalHelpers.clearBrowserState.

fixes #1669
Merge pull request #1674 from mozilla/issue-1669-webchannel-functional-tests

fix(tests): Fix the oauth_webchannel tests when run by themselves.