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.
core: add wget and ca-certificates
unmount: Remove ' (deleted)' from /proc/mount entries.

Try removing it, or not, in case somebody wants a mount point ending in ' (deleted)'.
Do not run upgrade test if some of the releases are not being tested.
Test: Change /dev/urandom handling
 - Set CROUTON_WEAK_RANDOM in test/run.sh

If CROUTON_WEAK_RANDOM is set:
 - mount-chroot: Select urandom to generate encryption key.
 - Bind mount /dev/urandom to /dev/random in the chroot.
Merge pull request #536 from drinkcat/misc

Add wget/ca-certificates, change /dev/urandom handling + 2 minor fixes.
Arch installer.
Arch: source /etc/profile in enter-chroot.
Arch: Core and post-common targets.
Arch: x11/xephyr targets
Arch: Most DE and X11-related targets.
Arch: Chrome target (untested).