Merge lp:~mterry/duplicity/more-test-fixes into lp:duplicity/0.6

[Michael Terry]

Here are some test fixes I found when running duplicity's test suite in a chroot.

1) Sometimes backups would complete so fast that the last incremental backup would have an end timestamp of X and the next full backup would have timestamp of X as well (i.e. backups finished and started within a second). When this happened, collections.py would throw an exception because it doesn't expect that. So I added a sleep(1).

2) At one point, the tests back up /etc, and expect there to be more than one volume. On a poorly-populated chroot, /etc may fit under 1M when compressed. So back up /bin instead.

3) The test-other-filesystem support test assumed /dev was on another filesystem. This wasn't true in my chroot. So I made the test more robust in that case and added a check for /proc, which in my experience is more likely to be a mount.