Fix incorrect test where corrupted version was not really written.
Fix incorrect test where corrupted version was not really written.

debugfs won't overwrite existing files, so we need to delete it first
to write our corrupted version. Why this has been passing until now is
unknown, but with the new reboot detector, the old test is no longer
passing.

Changelog: None

Signed-off-by: Kristian Amlie <505e66ae45028a0596c853559221f0b72c1cee21@northern.tech>

GregorioDiStefano/integration,pasinskim/integration,pasinskim/integration,GregorioDiStefano/integration,pasinskim/integration