[step_runner] run _merge_envs in simulation too.

recipe_modules/context/examples/full.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.
 
 from recipe_engine import recipe_api, config
 
 DEPS = [
   'context',
   'path',
   'step',
@@ skipping 12 matching lines @@
   # can change cwd
   api.step('mk subdir', ['mkdir', 'subdir'])
   with api.context(cwd=api.path['start_dir'].join('subdir')):
     api.step('subdir step', ['bash', '-c', 'pwd'])
     api.step('other subdir step', ['bash', '-c', 'echo hi again!'])
 
   # can set envvars
   with api.context(env={"HELLO": "WORLD", "HOME": None}):
     api.step('env step', ['bash', '-c', 'echo $HELLO; echo $HOME'])
 
+  # %-formats are errors (for now). Double-% escape them.
+  bad_examples = ["%format", "%s"]
+  for example in bad_examples:
+    try:
+      with api.context(env={"BAD": example}):
+        assert False  # pragma: no cover
+    except ValueError:
+      pass
+
+  # this is fine though:
+  api.context(env={"FINE": "%%format"})
+
   # can increment nest level... note that this is a low level api, prefer
   # api.step.nest instead:
   # YES:
   with api.step.nest('nested'):
     api.step('properly indented', ['bash', '-c', 'echo yay!'])
   # AVOID:
   with api.context(increment_nest_level=True):
     api.step('indented with wrong name', ['bash', '-c', 'echo indent?'])
 
 
 def GenTests(api):
   yield api.test('basic')