[Perf] Update references to telemetry.core.discover

tools/perf/benchmarks/benchmark_unittest.py

 # Copyright 2014 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.
 
 """For all the benchmarks that set options, test that the options are valid."""
 
 from collections import defaultdict
 import unittest
 
 from core import path_util
 from core import perf_benchmark
 
 from telemetry import benchmark as benchmark_module
 from telemetry import decorators
-from telemetry.core import discover
 from telemetry.internal.browser import browser_options
 from telemetry.testing import progress_reporter
 
+from py_utils import discover
 
 def _GetAllPerfBenchmarks():
   return discover.DiscoverClasses(
       path_util.GetPerfBenchmarksDir(), path_util.GetPerfDir(),
       benchmark_module.Benchmark, index_by_class_name=True).values()
 
 
 def _BenchmarkOptionsTestGenerator(benchmark):
   def testBenchmarkOptions(self):  # pylint: disable=unused-argument
     """Invalid options will raise benchmark.InvalidOptionsError."""
@@ skipping 68 matching lines @@
     suite.addTest(BenchmarkOptionsTest(benchmark.Name()))
 
 
 def load_tests(loader, standard_tests, pattern):
   del loader, pattern  # unused
   suite = progress_reporter.TestSuite()
   for t in standard_tests:
     suite.addTests(t)
   _AddBenchmarkOptionsTests(suite)
   return suite