perf tools: Add debug messages and comments for testing

Add debug messages to enable scripts to track aspects of 'perf record'
behaviour. The messages will be consumed after 'perf record' has run,
with the exception of "perf record has started" which is consequently
flushed.

Put comments so developers know which messages are also being used by test
scripts.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20220912083412.7058-11-adrian.hunter@intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>