Webform user test cleanup

 - Renamed the existing test case classes to have the "TestCase" suffix.
 - Renamed WebformUserSetup to WebformUserTestSetup and renamed its file to webform_user_test_setup.php to distinguish it from the test cases.
 - Updated the .info file with the new filename.
 - Made WEbformUserTestSetup abstract so it won't try to run as a test.
 - Cleaned up comment.s
 - Added todo labels where tests looked incomplete.
 - Added the missing field_truncate test to the .info file so Simpletest can find it.