client: Split DockerClient init unittests into separate class
client: Split DockerClient init unittests into separate class

This allows for using setUp()/tearDown() for doing client initialization
and more.

Signed-off-by: Esben Haabendal <da90c138e4a9573086862393cde34fa33d74f6e5@haabendal.dk>

XD-embedded/xd-docker,esben/xd-docker,esben/xd-docker,XD-embedded/xd-docker