Track step nesting in StreamEngine.

recipe_modules/generator_script/example.py

 # Copyright 2013 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.
 
 from recipe_engine.recipe_api import Property
 
 DEPS = [
   'generator_script',
   'json',
   'path',
@@ skipping 55 matching lines @@
       {'name': 'runs', 'cmd': ['echo', 'runs succeeds']},
       {'name': 'fails', 'cmd': ['echo', 'fails fails!']},
       {'name': 'skipped', 'cmd': ['echo', 'absent']},
       {'name': 'always_runs', 'cmd': ['echo', 'runs anyway'],
        'always_run': True},
     ) +
     api.step_data('fails', retcode=1)
   )
 
   yield (
+    api.test('nested') +
+    api.properties(script_name='nested.py') +
+    api.generator_script(
+      'nested.py',
+      {'name': 'grandparent', 'cmd': ['echo', 'grandparent']},
+      {'name': 'parent', 'step_nest_level': 1, 'cmd': ['echo', 'parent']},
+      {'name': 'child', 'step_nest_level': 2, 'cmd': ['echo', 'child']},
+      {'name': 'sibling', 'step_nest_level': 2, 'cmd': ['echo', 'sibling']},
+      {'name': 'uncle', 'step_nest_level': 1, 'cmd': ['echo', 'uncle']},
+      {'name': 'cousin', 'step_nest_level': 2, 'cmd': ['echo', 'cousin']},
+    )
+  )
+
+  yield (
     api.test('malformed_command') +
     api.properties(script_name='malformed.py') +
     api.generator_script(
       'malformed.py',
       {'name': 'run', 'cmd': ['echo', 'there are', 4, 'cows']}) +
     api.expect_exception('AssertionError')
   )