zdtm: clean a test root before executing tests in userns

A tests is executed from different users in cases with and without
userns, so it can't to open files which were created before.

Here is an example for ns/user/static/inotify_irmap:
13355 mkdir("/etc", 0600)               = -1 EEXIST (File exists)
13355 unlink("/etc/zdtm-test")          = -1 EACCES (Permission denied)
13355 creat("/etc/zdtm-test", 0600)     = -1 EACCES (Permission denied)

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>