selftests: mptcp: use += operator to append strings

This patch uses addition assignment operator (+=) to append strings
instead of duplicating the variable name in mptcp_connect.sh and
mptcp_join.sh.

This can make the statements shorter.

Note: in mptcp_connect.sh, add a local variable extra in do_transfer to
save the various extra warning logs, using += to append it. And add a
new variable tc_info to save various tc info, also using += to append it.
This can make the code more readable and prepare for the next commit.

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/20240308-upstream-net-next-20240308-selftests-mptcp-unification-v1-8-4f42c347b653@kernel.org


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