autopkgtest: increase timeout to 240s on s390x

Benjamin Drungrequested to merge
bdrung/initramfs-tools:s390x-timeout into debian/latest

The Ubuntu s390x autopkgtest currently runs into the 120 seconds timeout. It times out after printing the virtio_net rename log lines. The autopkgtest on Debian only need 18 seconds to reach that log line.

Double the timeout from 120s to 240s on s390x.

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