CI: run slirp4netns tests
The test scripts of slirp4netns might be useful for testing libslirp itself: https://github.com/rootless-containers/slirp4netns/tree/master/tests
e.g., DHCP regression #48 (closed) could be detected with test-slirp4netns-dhcp.sh
(being added in https://github.com/rootless-containers/slirp4netns/pull/270)