Enable strict coverage for step module

recipes/example/nested.py

-# Copyright 2015 The LUCI Authors. All rights reserved.
-# Use of this source code is governed under the Apache License, Version 2.0
-# that can be found in the LICENSE file.
-
-
-DEPS = [
-  'step',
-]
-
-
-def RunSteps(api):
-  # Nest all steps below this.
-  with api.step.nest('complicated thing'):
-    with api.step.nest('first part'):
-      api.step('wait a bit', ['sleep', '1'])
-
-    # Prefix the name without indenting.
-    with api.step.context({'name': 'attempt number'}):
-      step_result = api.step('one', ['echo', 'herpy'])
-      assert step_result.step['name'] == 'complicated thing.attempt number.one'
-      api.step('two', ['echo', 'derpy'])
-
-  # Outer nested step's status should not inherit from inner.
-  with api.step.nest('inherit status') as nest_step:
-    with api.step.nest('inner step') as other_nest_step:
-      other_nest_step.presentation.status = api.step.EXCEPTION
-  assert nest_step.presentation.status == api.step.SUCCESS
-
-  # Change outer status after nesting is complete.
-  with api.step.nest('versatile status') as nest_step:
-    with api.step.nest('inner step'):
-      with api.step.nest('even deeper'):
-        pass
-    nest_step.presentation.status = api.step.FAILURE
-  assert nest_step.presentation.status == api.step.FAILURE
-
-  api.step('simple thing', ['sleep', '1'])
-
-
-def GenTests(api):
-  yield api.test('basic')