autopkgtest: Set allow-stderr to ignore .X11-unix warnings

An alternate is to create the directory as is done in mutter's debian/rules & debian/tests/ensure-x11-dir

That still emits an error about the directory not being owned by root. mutter's commit history points out that the build does not have permission to set that owner. I assume that's true for autopkgtests as well unless we want to use needs-root or needs-sudo. Those restrictions appear to be supported in most places now except that I think they don't work with sbuild's schroot backend (although the unshare backend is recommended instead now.)

Closes: https://bugs.debian.org/1090910