perf test: Stat output per thread of just the parent process

Per-thread mode requires either system-wide (-a), a pid (-p) or a tid
(-t).

The stat output tests were using system-wide mode but this is racy when
threads are starting and exiting - something that happens a lot when
running the tests in parallel (perf test -p).

Avoid the race conditions by using pid mode with the pid of the parent
process.

Signed-off-by: Ian Rogers <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Athira Rajeev <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Disha Goel <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: James Clark <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: K Prateek Nayak <[e-mail]>
Cc: Kajol Jain <[e-mail]>
Cc: Kan Liang <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Song Liu <[e-mail]>
Cc: Tim Chen <[e-mail]>
Cc: Yicong Yang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/tests/shell/lib/stat_output.sh

···
 		echo "[Skip] paranoid and not root"
 		return
 	fi
-	perf stat --per-thread -a $2 true
+	perf stat --per-thread -p $$ $2 true
 	commachecker --per-thread
 	echo "[Success]"
 }

tools/perf/tests/shell/stat+json_output.sh

···
 		echo "[Skip] paranoia and not root"
 		return
 	fi
-	perf stat -j --per-thread -a -o "${stat_output}" true
+	perf stat -j --per-thread -p $$ -o "${stat_output}" true
 	$PYTHON $pythonchecker --per-thread --file "${stat_output}"
 	echo "[Success]"
 }