Increase AUM timeout

Currently this test fails due to timeout.

Generation of signed and non-signed static delta in aum-offline-upgrade/run-test-signature.sh take long time.

In apertis-test-cases/test-cases/aum-offline-upgrade-signed.yaml, run-test-in-systemd's timeout is already set to 900 secs (15 min).

Signed-off-by: Frédéric Danis frederic.danis@collabora.com