tests/eas/acceptance: increase accuracy of little cpus test in the Of…

…fload Migration testcase

The window for test_little_cpus_run_tasks() was loosely defined as just
the migrator delay, which doesn't take into account the time it takes
for rt-app to spawn and create its tasks.  After
fa4bce7 ("tests/eas/acceptance: Increase required busy time for
idle pull test"), the test is more strict, so increase the accuracy of
the window to meet the new restrictions.

tests/eas/acceptance.py

@@ -400,6 +400,12 @@ def calculate_end_times(cls):
 
         return end_times
 
+    def get_migrator_activation_time(self):
+        start_times_dict = self.m_assert.getStartTime()
+        start_time = min(t['starttime'] for t in start_times_dict.itervalues())
+
+        return start_time + OFFLOAD_MIGRATION_MIGRATOR_DELAY
+
     def test_first_cpu_early_starters(self):
         """Offload Migration and Idle Pull: Test First CPU (Early Starters)"""
 
@@ -470,7 +476,7 @@ def test_little_cpus_run_tasks(self):
             if not first_task_finish_time or (end_time < first_task_finish_time):
                 first_task_finish_time = end_time
 
-        window = (OFFLOAD_MIGRATION_MIGRATOR_DELAY, first_task_finish_time)
+        window = (self.get_migrator_activation_time(), first_task_finish_time)
         busy_time = self.a_assert.getCPUBusyTime("cluster",
                                                  self.env.target.bl.littles,
                                                  window=window)