Allow non-seccomp platforms to pass integration-cli tests

Since seccomp is still a configurable build-tag, add a requirements
entry for seccomp, as well as move seccomp tests to "_unix" given it
won't be applicable to other platforms at this time.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)