selftests/bpf: Add BPF_FIB_LOOKUP_SRC tests

This patch extends the existing fib_lookup test suite by adding two test
cases (for each IP family):

* Test source IP selection from the egressing netdev.
* Test source IP selection when an IP route has a preferred src IP addr.

Signed-off-by: Martynas Pumputis <m@lambda.lt>
Link: https://lore.kernel.org/r/20231007081415.33502-3-m@lambda.lt


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>