mv contrib/cmd tests/cmd (except memfd-bind)

The following commands are moved from `contrib/cmd` to `tests/cmd`:
- fs-idmap
- pidfd-kill
- recvtty
- remap-rootfs
- sd-helper
- seccompagent

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

.gitignore

@@ -1,13 +1,13 @@
 vendor/pkg
 /runc
 /runc-*
-/contrib/cmd/recvtty/recvtty
-/contrib/cmd/sd-helper/sd-helper
-/contrib/cmd/seccompagent/seccompagent
-/contrib/cmd/fs-idmap/fs-idmap
 /contrib/cmd/memfd-bind/memfd-bind
-/contrib/cmd/pidfd-kill/pidfd-kill
-/contrib/cmd/remap-rootfs/remap-rootfs
+/tests/cmd/recvtty/recvtty
+/tests/cmd/sd-helper/sd-helper
+/tests/cmd/seccompagent/seccompagent
+/tests/cmd/fs-idmap/fs-idmap
+/tests/cmd/pidfd-kill/pidfd-kill
+/tests/cmd/remap-rootfs/remap-rootfs
 man/man8
 release
 Vagrantfile