selftests: mptcp: join: mark 'fastclose' 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, and if a debug kernel config is
being used.

It looks like this issue is often present with the NetDev CI. While this
is being investigated, the tests are marked as flaky not to create
noises on such CIs.

Fixes: 01542c9b

 ("selftests: mptcp: add fastclose testcase")
Link: https://github.com/multipath-tcp/mptcp_net-next/issues/324
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-v1-3-a352362f3f8e@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>