unittests: fail early when low on disk

Scripts from qa that are run as unittests via test/vstart_wrapper.sh may
fail because the partition on which it runs is low on space ( 95% full
). When it happens the cause of the problem may be unclear because it
is likely to show only as a client not being able to reach the mon.

A test is added in test/vstart_wrapper.sh to verify the disk space usage
using the same method as the mon would and fail with a detailed error if
it is the case.

Signed-off-by: Loic Dachary <loic@dachary.org>