volume subdir has no `integ-test` Makefile rule

Right now the top level Makefile uses a list of sub-directories to target running integration tests:

https://github.com/firecracker-microvm/firecracker-containerd/blob/279a03c70541cd355f4f4a8adda0480fbc6fb95d/Makefile#L14-L16

For example, the `volume` directory is included in this list, but its Makefile does not contain an `integ-test` rule, resulting in failure. The integration test(s) for volume live under the `snapshotter` directory.

Is there a reason why volume integ tests are under `snapshotter` ? If there is, we can just add a no-op `integ-test` rule to the volume Makefile. If not, maybe we can refactor to just pull that test into the volume directory.

e: answering the above question: volume is provided as a package and consumed in the volume integ test. moving this test to the volume dir would introduce an import cycle.