piuparts.py - Implement post-purge test of additional filesystem mounts.
piuparts.py - Unmount more than just proc.

Tests of e.g. jarwrapper are failing with recursion exceeded in
piuparts due to failure of /proc to unmount, because there is a
binfmt mount underneath it. Now that there is a test of new mounts,
let piuparts exit cleanly by removing all mounts.