selftests: net: include lib/sh/*.sh with lib.sh

selftests: net: include lib/sh/*.sh with lib.sh

Recently, the net/lib.sh file has been modified to include defer.sh from
net/lib/sh/ directory. The Makefile from net/lib has been modified
accordingly, but not the ones from the sub-targets using net/lib.sh.

Because of that, the new file is not installed as expected when
installing the Forwarding, MPTCP, and Netfilter targets, e.g.

  # make -C tools/testing/selftests TARGETS=net/mptcp install \
        INSTALL_PATH=/tmp/kself
  # cd /tmp/kself/
  # ./run_kselftest.sh -c net/mptcp
    TAP version 13
    1..7
    # timeout set to 1800
    # selftests: net/mptcp: mptcp_connect.sh
    # ./../lib.sh: line 5: /tmp/kself/net/lib/sh/defer.sh: No such file
      or directory
    # (...)

This can be fixed simply by adding all the .sh files from net/lib/sh
directory to the TEST_INCLUDES variable in the different Makefile's.

Fixes: a6e263f125cd ("selftests: net: lib: Introduce deferred commands")
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20241104-net-next-selftests-lib-sh-deps-v1-1-7c9f7d939fc2@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>