selftests: net: included needed helper in the install targets

The blamed commit below introduce a dependency in some net self-tests
towards a newly introduce helper script.

Such script is currently not included into the TEST_PROGS_EXTENDED list
and thus is not installed, causing failure for the relevant tests when
executed from the install dir.

Fix the issue updating the install targets.

Fixes: 3bdd9fd2

 ("selftests/net: synchronize udpgro tests' tx and rx connection")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/076e8758e21ff2061cc9f81640e7858df775f0a9.1706131762.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>