Only import "testing" from *_test.go

This prevents the testing package flags from leaking into the
flagsets of binaries that import libcontainer.

Docker-DCO-1.1-Signed-off-by: Peter Bourgon <peter@bourgon.org> (github: peterbourgon)
Merge pull request #135 from peterbourgon/master

Only import "testing" from *_test.go