selftest: Add test for bind() conflicts.

The test checks if (IPv4, IPv6) address pair properly conflict or not.

  * IPv4
    * 0.0.0.0
    * 127.0.0.1

  * IPv6
    * ::
    * ::1

If the IPv6 address is [::], the second bind() always fails.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>