Add Cristian as maintainer to pkg/testutils

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)

pkg/testutils/MAINTAINERS

@@ -1 +1,2 @@
 Solomon Hykes <s@docker.com> (@shykes)
+Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)