tests: Clean up 'test_user'

tests: Clean up 'test_user'

* Don't use hardcode routes: use the reverse function instead
* Make use of 'create_' helper functions
* Minimize duplication of code
* Remove unneeded 'XXX.objects.delete()' calls (all objects are deleted
  on teardown of each test)
* Include every import on its own line
* Use underscore_case, rather than camelCase

This includes one trivial, albeit necessary, removal of an import from
'test_user_browser'.

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Reviewed-by: Andy Doan <andy.doan@linaro.org>