perf bench: Use unbuffered output when pipe/tee'ing to a file

The output of 'perf bench' gets buffered when I pipe it to a file or to
tee, in such a way that I can see it only at the end.

E.g.

  $ perf bench internals synthesize -t
  < output comes out fine after each test run >

  $ perf bench internals synthesize -t | tee file.txt
  < output comes out only at the end of all tests >

This patch resolves this issue for 'bench' and 'test' subcommands.

See, also:

  $ perf bench mem all | tee file.txt
  $ perf bench sched all | tee file.txt
  $ perf bench internals all -t | tee file.txt
  $ perf bench internals all | tee file.txt

Committer testing:

It really gets staggered, i.e. outputs in bursts, when the buffer fills
up and has to be drained to make up space for more output.

Suggested-by: Riccardo Mancini <rickyman7@gmail.com>
Signed-off-by: Sohaib Mohamed <sohaib.amhmd@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Fabian Hemmer <copy@copy.sh>
Cc: Ian Rogers <irogers@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20211119061409.78004-1-sohaib.amhmd@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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