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