selftests: mptcp: diag: remove trailing whitespace

It doesn't need to be there, and it can cause some issues with TAP
parsers expecting only one space around the directive delimiter (#).

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240906-net-next-mptcp-ksft-subtest-time-v2-4-31d5ee4f3bdf@kernel.org


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