selftests: mptcp: join: change capture/checksum as bool

selftests: mptcp: join: change capture/checksum as bool

To maintain consistency with other scripts, this patch changes vars
'capture' and 'checksum' as bool vars in mptcp_join.

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://lore.kernel.org/r/20240223-upstream-net-next-20240223-misc-improvements-v1-7-b6c8a10396bd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>