Allow stderr for python3-gi autopkgtests

mesa 24 is now logging

MESA: warning: ZINK: vkCreateInstance failed (VK_ERROR_INCOMPATIBLE_DRIVER)

The mesa developers would like to keep emitting that error in case there is an actual issue with the driver and would prefer not to change that behavior

https://gitlab.freedesktop.org/mesa/mesa/-/issues/10293

This would fix the autopkgtest regression triggered by mesa 24 in Experimental

https://release.debian.org/britney/pseudo-excuses-experimental.html#mesa

cc/ @smcv Does this seem reasonable?

The mesa developers would like to keep emitting that error in case there is an actual issue with the driver

Having all GL applications complain about Zink (before falling back to llvmpipe and working fine) seems like an anti-pattern, but whatever. This change seems fine, we are not intentionally being extra-twitchy about "anything on stderr is fatally bad".

merged