selftests/bpf: Test BPF_PROG_ASSOC_STRUCT_OPS command

selftests/bpf: Test BPF_PROG_ASSOC_STRUCT_OPS command

Test BPF_PROG_ASSOC_STRUCT_OPS command that associates a BPF program
with a struct_ops. The test follows the same logic in commit
ba7000f1c360 ("selftests/bpf: Test multi_st_ops and calling kfuncs from
different programs"), but instead of using map id to identify a specific
struct_ops, this test uses the new BPF command to associate a struct_ops
with a program.

The test consists of two sets of almost identical struct_ops maps and BPF
programs associated with the map. Their only difference is the unique
value returned by bpf_testmod_multi_st_ops::test_1().

The test first loads the programs and associates them with struct_ops
maps. Then, it exercises the BPF programs. They will in turn call kfunc
bpf_kfunc_multi_st_ops_test_1_prog_arg() to trigger test_1() of the
associated struct_ops map, and then check if the right unique value is
returned.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20251203233748.668365-5-ameryhung@gmail.com