tests: remap rootfs for userns tests

Previously, all of our userns tests worked around the remapping issue by
creating the paths that runc would attempt to create (like /proc).
However, this isn't really accurate to how real userns containers are
created, so it's much better to actually remap the rootfs.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

.gitignore

@@ -7,6 +7,7 @@ vendor/pkg
 /contrib/cmd/fs-idmap/fs-idmap
 /contrib/cmd/memfd-bind/memfd-bind
 /contrib/cmd/pidfd-kill/pidfd-kill
+/contrib/cmd/remap-rootfs/remap-rootfs
 man/man8
 release
 Vagrantfile