selftests/net: ensure mptcp is enabled in netns

selftests/net: ensure mptcp is enabled in netns

Some distributions may not enable MPTCP by default. All other MPTCP tests
source mptcp_lib.sh to ensure MPTCP is enabled before testing. However,
the ip_local_port_range test is the only one that does not include this
step.

Let's also ensure MPTCP is enabled in netns for ip_local_port_range so
that it passes on all distributions.

Suggested-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250224094013.13159-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>