tests: use testify in oci_test.go
tests: use testify in oci_test.go

testify has a good api making test clear and easier to
understand

Signed-off-by: Julio Montes <4a43a13347431ef00be7d745d08a5ba1757e840b@intel.com>

clearcontainers/runtime,devimc/runtime,chavafg/runtime,clearcontainers/runtime,chavafg/runtime,devimc/runtime