llvmpipe: Bump test timeout to 180 seconds
120 still wasn't always enough for the s390x cross-build job, see e.g. https://gitlab.freedesktop.org/mesa/mesa/-/jobs/1551685
120 still wasn't always enough for the s390x cross-build job, see e.g. https://gitlab.freedesktop.org/mesa/mesa/-/jobs/1551685