The commit e8009f41 ('box: user.grant
error should be versatile') did not do proper clean-up: it grants
non-default privileges for user 'guest' and does not revoke them at the
end. That caused occasional failures of other tests, all with the same
error saying user 'guest' already had access on universe.

This case should be handled by test-run in a future, see [1].

[1]: https://github.com/tarantool/test-run/issues/156

Follows up #714