tests: net: tcp: Add testcase for validating retransmitted FIN seq

Implemented testcase to check if the retransmitted FIN packet
from the FIN1 state has the correct sequence number.
Also fix the endianness conversion of sequence numbers in the
tests to use 32-bit as it should instead of 16-bit.

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>