harness-perf: Use -o only for perf record

[k0kubun]

The -o option in harness-perf is useful because only some benchmarks use Dir.chdir and you might end up reading perf.data of other benchmarks without it. So it's useful for perf record, however, I want the output of perf stat to go to stdout as it normally does.

This PR changes the harness to add -o option only when it's used with PERF=record.