perf test: Be tolerant of missing json metric none value

print_metric_only_json and print_metric_end in stat-display.c may
create a metric value of "none" which fails validation as isfloat. Add
a helper to properly validate metric numeric values.

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

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

···
 def is_counter_value(num):
   return isfloat(num) or num == '<not counted>' or num == '<not supported>'
 
+def is_metric_value(num):
+  return isfloat(num) or num == 'none'
+
 def check_json_output(expected_items):
   checks = {
       'counters': lambda x: isfloat(x),
···
       'event-runtime': lambda x: isfloat(x),
       'interval': lambda x: isfloat(x),
       'metric-unit': lambda x: True,
-      'metric-value': lambda x: isfloat(x),
+      'metric-value': lambda x: is_metric_value(x),
       'metric-threshold': lambda x: x in ['unknown', 'good', 'less good', 'nearly bad', 'bad'],
       'metricgroup': lambda x: True,
       'node': lambda x: True,