zdtm.py: bind-mount a test root as slave

In this case it has an external master_id and we want to test that
criu can handle it correctly.
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm.py

@@ -174,7 +174,7 @@ class ns_flavor:
 		self.__mknod("rtc")
 
 	def init(self, l_bins, x_bins):
-		subprocess.check_call(["mount", "--make-private", "--bind", ".", self.root])
+		subprocess.check_call(["mount", "--make-slave", "--bind", ".", self.root])
 		self.root_mounted = True
 
 		if not os.access(self.root + "/.constructed", os.F_OK):