selftests/bpf: Add test cases for struct_ops prog

Running a BPF_PROG_TYPE_STRUCT_OPS prog for dummy_st_ops::test_N()
through bpf_prog_test_run(). Four test cases are added:
(1) attach dummy_st_ops should fail
(2) function return value of bpf_dummy_ops::test_1() is expected
(3) pointer argument of bpf_dummy_ops::test_1() works as expected
(4) multiple arguments passed to bpf_dummy_ops::test_2() are correct

Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20211025064025.2567443-5-houtao1@huawei.com