testutils: newNS() works in a rootless user namespace

When running in a user namespace created by an unprivileged user the
owner of /var/run will be reported as the unknown user (as defined in
/proc/sys/kernel/overflowuid) so any access to the directory will
fail.

If the XDG_RUNTIME_DIR environment variable is set, check whether the
current user is also the owner of /var/run.  If the owner is different
than the current user, use the $XDG_RUNTIME_DIR/netns directory.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

.travis.yml

@@ -1,6 +1,6 @@
 language: go
 sudo: required
-dist: trusty
+dist: xenial
 
 go:
   - 1.11.x