zdtm.sh: use tmpfs for dump files

It's optional. It's going to be used in jenkins.

time bash -x test//zdtm.sh  static/maps04
real	0m40.220s
user	0m0.096s
sys	0m12.822s

time bash -x test//zdtm.sh -t static/maps04
real	0m9.904s
user	0m0.074s
sys	0m1.630s

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