Update tests to run with Docker containerd snapshotter enabled by default.

Previously, the Sysbox test container was starting Docker engine inside
the container with containerd-snapshotter disabled and running most of
the tests this way. Only a few tests were run with containerd-snapshotter
enabled.

Switch this to the opposite scenario: make the tests run with Docker's
containerd-snapshotter enabled by default, and run a subset of them with
containerd-snapshotter disabled (see test_with_docker_image_store() in
testSysbox).

Note that this change required minor changes in existing tests due
to the tests being originally written with the assumption that
Docker was using the docker image store (containerd-snapshotter disabled).

Signed-off-by: Cesar Talledo <cesar.talledo@docker.com>