Expose testutil integration workers as public

[jedevc]

This PR exposes the integration test workers as public, performing a few small refactors to make them more flexible (so that the caller can always enforce a custom name).

This is designed to be used in the context of docker/buildx#1770, where we wrap the moby worker to create the docker driver worker, and wrap the oci worker to create the remote driver worker.

This allows us to avoid copying all the logic to spin up buildkit sandboxes inside of buildx, and to share that logic, so that we can automatically consume changes from the upstream test framework.

Additionally, we also add a new DockerAddress method to the Backend interface which returns the address of the UNIX socket for worker backends that support it (currently only moby).

type Backend interface {
	Address() string
+	DockerAddress() string
	ContainerdAddress() string
   ...

This is designed to be consumed by the buildx docker driver worker wrapper, so that we can connect directly to the docker instance using the docker driver.

[tonistiigi]

This could maybe use some future refactoring if we want to make integration pkg more reusable. Currently, some of these methods are very specific for a single worker implementation (or "buildkit worker" implementation). Ideally this could be used for any backend. Now that worker implementations are public they don't need to be always defined and imported with this pkg either.

Otoh, we need to be careful that the test sides do not need a lot of boilerplate for type checking.