Fix some flaky tests related to pick port (#985)

* Fix some tests to PickPort correctly

* Add retry logic for two socks tests

Two socks tests listen for both TCP and UDP with same port number, in some cases the port is not available
Add retry logic for server core, start server core and client core separately
Extract a common method

* Add retry logic for two dokodemo tests

Two dokodemo tests listen for 5 ports, in some cases they are not available
Add retry logic for client core, start server and client separately