selftests: mptcp: include net_helper.sh file

Similar to the previous commit, the net_helper.sh file from the parent
directory is used by the MPTCP selftests and it needs to be present when
running the tests.

This file then needs to be listed in the Makefile to be included when
exporting or installing the tests, e.g. with:

  make -C tools/testing/selftests \
          TARGETS=net/mptcp \
          install INSTALL_PATH=$KSFT_INSTALL_PATH

  cd $KSFT_INSTALL_PATH
  ./run_kselftest.sh -c net/mptcp

Fixes: 1af3bc91 ("selftests: mptcp: lib: use wait_local_port_listen helper")
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240910-net-selftests-mptcp-fix-install-v1-3-8f124aa9156d@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>