selftests/bpf: Fix unintentional switch case fall through

Break from switch expression after parsing -n CLI argument in veristat,
instead of falling through and enabling comparison mode.

Fixes: a5c57f81 ("veristat: add ability to set BPF_F_TEST_SANITY_STRICT flag with -r flag")
Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20250617121536.1320074-1-mykyta.yatsenko5@gmail.com