selftests: mptcp: join: simplify checksum_tests

The four checksum tests are similar, only one line is different. So
a for-loop can be used to simplify these tests.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240902-net-next-mptcp-mib-mpjtx-misc-v1-10-d3e0f3773b90@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>