selftests/bpf: make veristat's verifier log parsing faster and more robust

selftests/bpf: make veristat's verifier log parsing faster and more robust

Make sure veristat doesn't spend ridiculous amount of time parsing
verifier stats from verifier log, especially for very large logs or
truncated logs (e.g., when verifier returns -ENOSPC due to too small
buffer). For this, parse lines from the end of the log and make sure we
parse only up to 100 last lines, where stats should be, if at all.

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220923175913.3272430-3-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>