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 <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

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

@@ -181,7 +181,7 @@ check_metric_only()
 		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]"
 }