selftests/bpf: Test a BPF CC writing sk_pacing_*

selftests/bpf: Test a BPF CC writing sk_pacing_*

Test whether a TCP CC implemented in BPF is allowed to write
sk_pacing_rate and sk_pacing_status in struct sock. This is needed when
cong_control() is implemented and used.

Signed-off-by: Jörn-Thorben Hinz <jthinz@mailbox.tu-berlin.de>
Link: https://lore.kernel.org/r/20220622191227.898118-4-jthinz@mailbox.tu-berlin.de
Signed-off-by: Alexei Starovoitov <ast@kernel.org>