r/autopktest: get_max_priority_from_policy supports priorities < 100

On IRC I was notified that glibc very often tempfails for experimental runs. After a bit of debugging, I found out that the needle for the Version table only contained the first line with the version from experimental, 2.40-1:

libc6-dev-x32: Installed: (none) Candidate: 2.39-6 Version table: 2.40-1 1 1 http://deb.debian.org/debian experimental/main amd64 Packages 2.39-6 500 500 http://deb.debian.org/debian unstable/main amd64 Packages

Quoting apt_preferences(5): Priorities (P) assigned in the APT preferences file must be positive or negative integers. So, get_max_priority_from_policy needs to allow for 2 more spaces.