Test infrastructure: bind mount urandom over random

Avoids blocking on the entropy pool.
Obviously insecure, but it doesn't matter for running tests.
Add encryption test.
Improve password automation for tests.
Fix moving multiple chroots' key files
Improve unencrypted file listing when mounting a chroot.
Don't clean up $ROOT on unmount; there's no real benefit and it's a race.
edit-chroot: Fix spinner to check for command's exit status.
Fix locking of bootstrap/snapshot tarballs.
Clean up /var/run/crouton when all tests are finished.