selftest: net: Add test for TCP flow failover with ECMP routes.

selftest: net: Add test for TCP flow failover with ECMP routes.

Without the previous commit, TCP failed to switch to alternative
IPv6 routes immediately upon carrier loss.

It would persist with the dead route until reaching the threshold
net.ipv4.tcp_retries1, leading to unnecessary delays in failover.

Let's add a selftest for this scenario to ensure TCP fails over
immediately upon a carrier loss event.

Before:
  TEST: TCP IPv4 failover                                             [ OK ]
  TEST: TCP IPv6 failover                                             [FAIL]

After:
  TEST: TCP IPv4 failover                                             [ OK ]
  TEST: TCP IPv6 failover                                             [ OK ]

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Sagarika Sharma <sharmasagarika@google.com>
Link: https://patch.msgid.link/20260430200909.527827-3-sharmasagarika@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>