selftests: mptcp: reset the last TS before the first test

Just to slightly improve the precision of the duration of the first
test.

In mptcp_join.sh, the last append_prev_results is now done as soon as
the last test is over: this will add the last result in the list, and
get a more precise time for this last test.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240906-net-next-mptcp-ksft-subtest-time-v2-3-31d5ee4f3bdf@kernel.org


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