zdtm: Unlink link_remap files once dump complete

In case if only "dump" procedure is invoked we left service
"link_remap.%d" file(s) on disk. This prevents next "dump"
procedure of same test to fail because we hit the situation
where same named "link_remap.%d" already exist.

So, if test is passed with "-d" option we remove the link remap
file at the end.

Also with this patch we start passing --link-remap option
only for tests which have "unlink_" prefix in name. This applies
some limitation on how test could be named, thus maybe we need
more flexible solution here.

https://bugzilla.openvz.org/show_bug.cgi?id=2489



Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>