[telemetry] Create BrowserTestCase to reuse the browser for browser_unittest.

tools/perf/measurements/smoothness_unittest.py

 # Copyright 2013 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
 
 from measurements import smoothness
 from metrics import power
 from telemetry.core import exceptions
 from telemetry.core import wpr_modes
 from telemetry.page import page
 from telemetry.page import page_test
-from telemetry.testing import page_test_test_case
 from telemetry.unittest import options_for_unittests
+from telemetry.unittest import page_test_test_case
 
 class FakePlatform(object):
   def IsRawDisplayFrameRateSupported(self):
     return False
   def CanMonitorPower(self):
     return False
 
 
 class FakeBrowser(object):
   def __init__(self):
@@ skipping 138 matching lines @@
         self.fake_power = self._power_metric = FakePowerMetric(browser)
 
     measurement = BuggyMeasurement()
     try:
       self.RunMeasurement(measurement, ps)
     except page_test.TestNotSupportedOnPlatformFailure:
       pass
 
     self.assertTrue(measurement.fake_power.start_called)
     self.assertTrue(measurement.fake_power.stop_called)