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)
Graphtest is ok to compile normally

The graphtest package is only imported in the test files of other
packages therefore we do not leak testing flags.

Signed-off-by: Michael Crosby <michael@docker.com>
Merge pull request #7474 from crosbymichael/testing-import

Only import "testing" in test packages