autopkgtest: switch from xfvb-run to xwfb-run

happens to avoid stderr output with gtk4 4.20 as seen at https://ci.debian.net/packages/c/chatty/unstable/amd64/

Therefore, this fixes a blocker to getting gtk4 4.20 into Unstable/Testing

It may make sense to switch dh_auto_test to use xwfb-run too, but for now I just implemented the simplest fix/workaround for my issue.