PR go/59506

net: use DialTimeout in TestSelfConnect

Backported from master repository.

This avoids problems with systems that take a long time to
find out nothing is listening, while still testing for the
self-connect misfeature since a self-connect should be fast.
With this we may be able to remove the test for non-Linux
systems.

Tested (on GNU/Linux) by editing selfConnect in
tcpsock_posix.go to always return false and verifying that
TestSelfConnect then fails with and without this change.

Idea from Uros Bizjak.

From-SVN: r206224