selftest: tcp: Add bind() tests for SO_REUSEADDR/SO_REUSEPORT.

selftest: tcp: Add bind() tests for SO_REUSEADDR/SO_REUSEPORT.

This patch adds two tests using SO_REUSEADDR and SO_REUSEPORT and
defines errno for each test case.

SO_REUSEADDR/SO_REUSEPORT is set for the per-fixture two bind()
calls.

The notable pattern is the pair of v6only [::] and plain [::].
The two sockets are put into the same tb2, where per-bucket v6only
flag would be useless to detect bind() conflict.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20240326204251.51301-9-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>