Remove Unused local variable $username from /core/modules/user/lib/Drupal/user/Tests/Views/HandlerFieldUserNameTest.php

Part of meta-issue #2002650: [meta] improve maintainability by removing unused local variables

File /core/modules/user/lib/Drupal/user/Tests/Views/HandlerFieldUserNameTest.php

Line 39: Unused local variable $username
Line 46: Unused local variable $username
Line 58: Unused local variable $anon_name