selftests: net: Add FIN_ACK processing order related latency spike test

This commit adds a test for FIN_ACK process races related reconnection
latency spike issues.  The issue has described and solved by the
previous commit ("tcp: Reduce SYN resend delay if a suspicous ACK is
received").

The test program is configured with a server and a client process.  The
server creates and binds a socket to a port that dynamically allocated,
listen on it, and start a infinite loop.  Inside the loop, it accepts
connection, reads 4 bytes from the socket, and closes the connection.
The client is constructed as an infinite loop.  Inside the loop, it
creates a socket with LINGER and NODELAY option, connect to the server,
send 4 bytes data, try read some data from server.  After the read()
returns, it measure the latency from the beginning of this loop to this
point and if the latency is larger than 1 second (spike), print a
message.

Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/.gitignore

@@ -22,3 +22,4 @@ ipv6_flowlabel_mgr
 so_txtime
 tcp_fastopen_backup_key
 nettest
+fin_ack_lat