selftest: Update PATH for nettest in fcnal-test

Allow fcnal-test.sh to be run from top level directory in the
kernel repo as well as from tools/testing/selftests/net by
setting the PATH to find the in-tree nettest.

Signed-off-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240124214117.24687-2-dsahern@kernel.org


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