selftests: increase timeout to 10 min

On slow systems with kernel debug settings, we can reach the current
timeout when all tests are executed.

Likely some tests need be improved to remove some 'sleep' and wait
(less) for a specific action. This can also improve stability.
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=450
+timeout=600