selftests: mptcp: move stats info in case of errors to lib.sh

selftests: mptcp: move stats info in case of errors to lib.sh

A few MPTCP selftests are using the same code to print stats in case of
error. This code can then be moved to mptcp_lib.sh.

No behaviour changes intended, except to print the error in red and to
stderr, like most error messages.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250114-net-next-mptcp-st-more-debug-err-v1-3-2ffb16a6cf35@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>