zdtm: don't execute pipe00 a few times simultaneously

zdtm-pre-dump, zdtm-snapshot, zdtm-iter, zdtm execute pipe00, so
these targets should be executed one by one.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/Makefile

@@ -5,7 +5,11 @@ ZDTM_ARGS ?= -C
 
 .FORCE:
 
-all: zdtm zdtm-pre-dump zdtm-snapshot zdtm-iter
+all:
+	$(MAKE) zdtm
+	$(MAKE) zdtm-pre-dump
+	$(MAKE) zdtm-snapshot
+	$(MAKE) zdtm-iter
 
 .PHONY: all