Zeroconf discovery: sometimes, unregistrations are not detected

[tcalmant]

In some cases, services and frameworks unregistrations are not detected by the mDNS/Zeroconf service discovery. This leads to failures in Travis-CI builds, but can't be reproduced easily on my computers.

This could be due to:

• slow detection of the new service/framework unregistration (past the timeout)
• slow detection of the new service/framework registration (both events could be handled too close to each other)
• issue with Travis-CI containers network (I can't reproduce the bug on my computers)
• other ?

[tcalmant]

This issue was due to a typo in tests, which was checking the unregistration of a service before the mDNS timeout.