[Perf] Update references to telemetry.core.discover

tools/perf/benchmarks/system_health_unittest.py

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import sys
 import unittest
 
 from benchmarks import system_health as system_health_benchmark
 from core import path_util
 from page_sets.system_health import system_health_stories
 from page_sets.system_health import system_health_story
 
 from telemetry import benchmark as benchmark_module
-from telemetry.core import discover
+
+from py_utils import discover
 
 
 def _GetAllSystemHealthBenchmarks():
   all_perf_benchmarks = discover.DiscoverClasses(
       path_util.GetPerfBenchmarksDir(), path_util.GetPerfDir(),
       benchmark_module.Benchmark,
       index_by_class_name=True).values()
   return [b for b in all_perf_benchmarks if
           sys.modules[b.__module__] == system_health_benchmark]
 
@@ skipping 31 matching lines @@
         system_health_stories.MobileSystemHealthStorySet().stories)
     for s in desktop_stories + mobile_stories:
       # Long running stories has their own way of collecting memory dumps,
       # so they explicitly override RunPageInteractions method.
       if s.name.startswith('long_running:'):
         continue
       self.assertEquals(s.__class__.RunPageInteractions,
           system_health_story.SystemHealthStory.RunPageInteractions,
           'Story %s overrides RunPageInteractions. Override _DidLoadDocument '
           'instead' % s.name)