fix(test): Fix `test-latest` functional tests.

Several problems:
* When running against remote servers, Selenium moves very quickly.
* The email index being searched for was not being passed to restmail, so it would return as long as one email existed.
* OAuth tests depended on each other
* Signing out of 123done was not done correctly and state would hang around.
* Clearing content server state was being done before the content server was loaded, which ended up in a 500 error.

What we do now:
* Simplify the OAuth sign in tests so there is no cross-test dependencies.
* In the OAuth sign in tests, use the FxaClient to pre-generate users. We are testing sign in, not sign up.
* Ensure the content server page is loaded before clearing its state.
* Ensure the user is logged out of 123done when clearing its state - do so by calling XHR requests directly.
* Ensure the index of the email we are waiting for makes it to restmail.

fixes #1690