Only import "testing" from *_test.go

This prevents the testing package flags from leaking into the flagsets
of binaries that import docker. I left integration-cli alone.

Docker-DCO-1.1-Signed-off-by: Peter Bourgon <peter@bourgon.org> (github: peterbourgon)

mount/mountinfo_linux_test.go

@@ -1,3 +1,5 @@
+// +build linux
+
 package mount
 
 import (