selftests: mptcp: print all error messages to stdout

Some error messages are printed to stderr while the others are printed
to 'stdout'. As part of the unification, this patch drop "1>&2" to let
all errors messages are printed to 'stdout'.

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-v1-1-4f42c347b653@kernel.org


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