test/Makefile: export ZDTM_ARGS

This variable is used from Makefile.zdtm
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/Makefile

 RM := rm -f --one-file-system
 
 ZDTM_ARGS ?= -C
+export ZDTM_ARGS
 
 .FORCE: