fix(testing): raise the install monitor timeout to 150 minutes

A full archsetup install with heavy AUR builds (vagrant and its git-cloned
installers) can run past the old 90-minute monitor cap on a slow mirror. When
that happened the run stopped monitoring mid-install and validated a
half-installed system, producing spurious late-step failures. Raise MAX_POLLS
from 180 to 300 (90 -> 150 minutes) so a slow-but-healthy install completes.

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