selftests/bpf: Add BPF_FIB_LOOKUP_MARK tests

selftests/bpf: Add BPF_FIB_LOOKUP_MARK tests

This patch extends the fib_lookup test suite by adding a few test
cases for each IP family to test the new BPF_FIB_LOOKUP_MARK flag
to the bpf_fib_lookup:

  * Test destination IP address selection with and without a mark
    and/or the BPF_FIB_LOOKUP_MARK flag set

Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240326101742.17421-3-aspsk@isovalent.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>