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)
Upstream-commit: 0433e3891532a9783b77d6b02c041bab359b0d91
Component: engine
