selftests/bpf: Test loading bpf-tcp-cc prog calling the kernel tcp-cc kfuncs

This patch adds a test to ensure all static tcp-cc kfuncs is visible to
the struct_ops bpf programs. It is checked by successfully loading
the struct_ops programs calling these tcp-cc kfuncs.

This patch needs to enable the CONFIG_TCP_CONG_DCTCP and
the CONFIG_TCP_CONG_BBR.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20240322191433.4133280-2-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>