selftests: mptcp: simult flows: mark 'unbalanced' tests as flaky

These tests are flaky since their introduction. This might be less or
not visible depending on the CI running the tests, especially if it is
also busy doing other tasks in parallel.

A first analysis shown that the transfer can be slowed down when there
are some re-injections at the MPTCP level. Such re-injections can of
course happen, and disturb the transfer, but it looks strange to have
them in this lab. That could be caused by the kernel having access to
less CPU cycles -- e.g. when other activities are executed in parallel
-- or by a misinterpretation on the MPTCP packet scheduler side.

While this is being investigated, the tests are marked as flaky not to
create noises in other CIs.

Fixes: 219d0499

 ("mptcp: push pending frames when subflow has free space")
Link: https://github.com/multipath-tcp/mptcp_net-next/issues/475
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240524-upstream-net-20240524-selftests-mptcp-flaky-v1-2-a352362f3f8e@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>