perf test stat+json: Improve metric-only testing

When testing metric-only, pass a metric to perf rather than expecting
a hard coded metric value to be generated.

Remove keys that were really metric-only units and instead don't
expect metric only to have a matching json key as it encodes metrics
as {"metric_name", "metric_value"}.

Signed-off-by: Ian Rogers <[e-mail]>
Signed-off-by: Namhyung Kim <[e-mail]>

tools/perf/tests/shell/lib/perf_json_output_lint.py

···
       'socket': lambda x: True,
       'thread': lambda x: True,
       'unit': lambda x: True,
-      'insn per cycle': lambda x: isfloat(x),
-      'GHz': lambda x: True,  # FIXME: it seems unintended for --metric-only
   }
   input = '[\n' + ','.join(Lines) + '\n]'
   for item in json.loads(input):
···
                            f' in \'{item}\'')
     for key, value in item.items():
       if key not in checks:
+        if args.metric_only:
+          continue
         raise RuntimeError(f'Unexpected key: key={key} value={value}')
       if not checks[key](value):
         raise RuntimeError(f'Check failed for: key={key} value={value}')

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

···
 		echo "[Skip] CPU-measurement counter facility not installed"
 		return
 	fi
-	perf stat -j --metric-only -e instructions,cycles -o "${stat_output}" true
+	perf stat -j --metric-only -M page_faults_per_second -o "${stat_output}" true
 	$PYTHON $pythonchecker --metric-only --file "${stat_output}"
 	echo "[Success]"
 }