selftests: forwarding: List helper scripts in TEST_FILES Makefile variable

Some scripts are not tests themselves; they contain utility functions used
by other tests. According to Documentation/dev-tools/kselftest.rst, such
files should be listed in TEST_FILES. Currently they are incorrectly listed
in TEST_PROGS_EXTENDED so rename the variable.

Fixes: c085dbfb ("selftests/net/forwarding: define libs as TEST_PROGS_EXTENDED")
Suggested-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Link: https://lore.kernel.org/r/20240131140848.360618-6-bpoirier@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/forwarding/Makefile

@@ -112,7 +112,7 @@ TEST_PROGS = bridge_fdb_learning_limit.sh \
 	vxlan_symmetric_ipv6.sh \
 	vxlan_symmetric.sh
 
-TEST_PROGS_EXTENDED := devlink_lib.sh \
+TEST_FILES := devlink_lib.sh \
 	ethtool_lib.sh \
 	fib_offload_lib.sh \
 	forwarding.config.sample \