Fix flaky test TestPullAllTagsFromCentralRegistry

This test was directly comparing lines of output from "docker images".
Sometimes, when busybox had been pushed to the hub recently, the
relative creation times would differ like this:

... obtained []string = []string{"busybox", "latest", "d9551b4026f0", "27", "minutes", "ago", "1.113", "MB"}
... expected []string = []string{"busybox", "latest", "d9551b4026f0", "26", "minutes", "ago", "1.113", "MB"}

Fixing by removing the time-since-creation fields from the comparison.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>