Update image pull tests

Update and migrate existing tests to the `DockerHubPullSuite`. Most
tests were preserved, but refactored and made more exhaustive. One test
was deliberately removed (`TestPullVerified`) as it is unreliable and
that the feature was obsoleted by content trust.

Move all trust related tests to `docker_cli_pull_trusted_test.go`.

Move tests depending on a local registry to `docker_cli_pull_local_test.go`.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>