Fixing the race check

[fclairamb]

I broke them with the latest changes around the removal of mutex.

Now I'm fixing them and adding them as part of the CI.

[asv]

funny, I did not get time to push similar fix. Maybe it's worth making a Listener private?

[fclairamb]

This is a good point. The http.Server doesn't expose its Listener, so why should we ?

Still... I think it should be in an other PR because:

• This is a different subject
• It forces us to transform the API. The test framework for example makes an intensive use of the Listener (to be able to start many instances of the server as the same by letting the system decide which port we should listen on).

[fclairamb]

I'm merging this as this is a minor change that improves quality.