selftests/bpf: Replace TCP CC string comparisons with bpf_strncmp

selftests/bpf: Replace TCP CC string comparisons with bpf_strncmp

The connect4_prog and bpf_iter_setsockopt tests duplicate the same
open-coded TCP congestion control string comparison logic. Since
bpf_strncmp() provides the same functionality, use it instead to
avoid repeated open-coded loops.

This change applies only to functional BPF tests and does not affect
the verifier performance benchmarks (veristat.cfg). No functional
changes intended.

Reviewed-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Hoyeon Lee <hoyeon.lee@suse.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20251115225550.1086693-5-hoyeon.lee@suse.com