selftests: mptcp: increase timeout to 20 minutes

With the increase number of tests, one CI instance, using a debug kernel
config and not recent hardware, takes around 10 minutes to execute the
slowest MPTCP test: mptcp_join.sh.

Even if most CIs don't take that long to execute these tests --
typically max 10 minutes to run all selftests -- it will help some of
them if the timeout is increased.

The timeout could be disabled but it is always good to have an extra
safeguard, just in case.

Please note that on slow public CIs with kernel debug settings, it has
been observed it can easily take up to 45 minutes to execute all tests
in this very slow environment with other jobs running in parallel.
The slowest test, mptcp_join.sh takes ~30 minutes in this case.

In such environments, the selftests timeout set in the 'settings' file
is disabled because this environment is known as being exceptionnally
slow. It has been decided not to take such exceptional environments into
account and set the timeout to 20min.
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/mptcp/settings

-timeout=600
+timeout=1200