Enable strict coverage for step module

recipe_modules/step/tests/stdio.py

+# Copyright 2017 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 = [
+  'raw_io',
+  'step',
+]
+
+
+def RunSteps(api):
+  # Read command's stdout and stderr.
+  step_result = api.step('echo', ['echo', 'Hello World'],
+      stdout=api.raw_io.output(),
+      stderr=api.raw_io.output())
+
+  # Pass stuff to command's stdin, read it from stdout.
+  step_result = api.step('cat', ['cat'],
+      stdin=api.raw_io.input_text(data='hello'),
+      stdout=api.raw_io.output('out'))
+
+  # Example of auto-mocking stdout. '\n' appended to mock 'echo' behavior.
+  step_result = api.step(
+      'automock',
+      ['echo', 'huh'],
+      stdout=api.raw_io.output('out'),
+      step_test_data=(
+          lambda: api.raw_io.test_api.stream_output('huh\n')))
+  assert step_result.stdout == 'huh\n'
+
+
+def GenTests(api):
+  yield api.test('basic')