selftests/bpf: Check bpf_cubic_acked() is called via struct_ops

selftests/bpf: Check bpf_cubic_acked() is called via struct_ops

Test bpf_tcp_ca (in test_progs) checks multiple tcp_congestion_ops.
However, there isn't a test that verifies functions in the
tcp_congestion_ops is actually called. Add a check to verify that
bpf_cubic_acked is actually called during the test.

Suggested-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Song Liu <song@kernel.org>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230919060258.3237176-3-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>