tests: make test_admin_users user_and_repo_group_fail() fixture more stable

When adding authentication_token() to log_user(), database session lifetime
will in some cases change:

test_admin_users test_delete_repo_group_err() use the
user_and_repo_group_fail() fixture.

Before, it got ObjectDeletedError when trying to delete a deleted RepoGroup and
moved on.

After changing log_user(), py.test would emit a warning:

kallithea/tests/functional/test_admin_users.py::TestAdminUsersController::()::test_delete_repo_group_err
.../site-packages/sqlalchemy/orm/persistence.py:1340: SAWarning: DELETE statement on table 'groups' expected to delete 1 row(s); 0 were matched. Please set confirm_deleted_rows=False within the mapper configuration to prevent this warning.
% (table.description, expected, rows_matched)

Instead, use RepoGroup.get_by_group_name to verify the group exists before
trying to delete it.