socket.test had a number of flaky problems:
- socket readiness expectation & read timeouts
- race conditions on socket shutdown in emulation test cases
- UDP datagrams losses on mac os
- excessive random port searches
Now they are solved. 127.0.0.1 is now used instead of 0.0.0.0 or
localhost to prevent wrong connections where appropriate. Socket test
is not fragile anymore.

Closes #4426
Closes #4451
Closes #4469

(cherry picked from commit 4137134c)