tests: replace integration test by py.test+namespace tests

tests: replace integration test by py.test+namespace tests

Relying on namespaces enable us to quickly run isolated instances of
lldpd without the need of virtual machines. Since the startup time is
quite fast (despite having to wait for lldpd to be "ready"), we can use
a classic unittest framework like py.test to run tests and get
appropriate reports. Tests can be run in parallel to overcome the
slowness induced by all those `time.sleep(2)`.