selftests: net: 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. Move those utility scripts to
TEST_FILES.

Fixes: 1751eb42 ("selftests: net: use TEST_PROGS_EXTENDED")
Fixes: 25ae948b ("selftests/net: add lib.sh")
Fixes: b99ac184 ("kselftests/net: add missed setup_loopback.sh/setup_veth.sh to Makefile")
Fixes: f5173fe3

 ("selftests: net: included needed helper in the install targets")
Suggested-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Link: https://lore.kernel.org/r/20240131140848.360618-5-bpoirier@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>