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) <[e-mail]>
Reviewed-by: Petr Machata <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/forwarding/Makefile

···
 	tc_common.sh
 
 TEST_INCLUDES := \
-	../lib.sh
+	../lib.sh \
+	$(wildcard ../lib/sh/*.sh)
 
 include ../../lib.mk

tools/testing/selftests/net/mptcp/Makefile

···
 
 TEST_FILES := mptcp_lib.sh settings
 
-TEST_INCLUDES := ../lib.sh ../net_helper.sh
+TEST_INCLUDES := ../lib.sh $(wildcard ../lib/sh/*.sh) ../net_helper.sh
 
 EXTRA_CLEAN := *.pcap
 

tools/testing/selftests/net/netfilter/Makefile

···
 TEST_FILES += packetdrill
 
 TEST_INCLUDES := \
-	../lib.sh
+	../lib.sh \
+	$(wildcard ../lib/sh/*.sh)