The last update to github.com/stretchr/testify makes `assert.Equal`
consider the type of its arguments. This commit makes this test pass
again by only testing for value equality using `assert.EqualValues`.
The background monitoring thread was using the DockerClient before we
had written our fake data. This commit stops the background thread from
running during tests.
Fixes#5611.
All images are tracked, when they were created and when they were last
used. FreeSpace() evicts these images in least recently used order,
breaking ties with oldest first.