selftests: mptcp: decrease BW in simult flows

When running the simult_flow selftest in slow environments -- e.g. QEmu
without KVM support --, the results can be unstable. This selftest
checks if the aggregated bandwidth is (almost) fully used as expected.

To help improving the stability while still keeping the same validation
in place, the BW and the delay are reduced to lower the pressure on the
CPU.

Fixes: 1a418cb8 ("mptcp: simult flow self-tests")
Fixes: 219d0499

 ("mptcp: push pending frames when subflow has free space")
Cc: stable@vger.kernel.org
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240131-upstream-net-20240131-mptcp-ci-issues-v1-6-4c1c11e571ff@kernel.org


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