Add happy path test on ContainerLogs
Add happy path test on ContainerLogs

Complete unit tests for `ContainerLogs` by adding happy path cases, and
complete potential errors coverage (with `Since` time parsing errors).

Signed-off-by: Vincent Demeester <87e1f221a672a14a323e57bb65eaea19d3ed3804@sbr.pm>

vdemeester/engine-api,jwhonce/engine-api,thaJeztah/engine-api,docker/engine-api