program-runtime: add LogCollector::new_ref_with_limit()
LogCollector::new_ref_with_limit(limit: Option<usize>) can be used to
initialize a collector with the given bytes limit. The limit can be None, in
which case logs are never truncated.
new_ref_with_limit(None) is used by cargo-run-bpf-tests so that the output of
cargo test --target=bpfel-unknown-unknown is not truncated.
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