selftests: mptcp: join: mark 'fail' tests as flaky

These tests are rarely unstable. It depends 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 sometimes present with the NetDev CI. While
this is being investigated, the tests are marked as flaky not to create
noises on such CIs.

Fixes: b6e074e1

 ("selftests: mptcp: add infinite map testcase")
Link: https://github.com/multipath-tcp/mptcp_net-next/issues/491
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-v1-4-a352362f3f8e@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>