cros_test_lib: Refactor cros_build_lib mocks

A large number of unit tests import cros_build_lib_unittest to get
access to RunCommandMock and PopenMock.  Let's refactor these helper
classes out to start breaking up all the weird dependencies.

BUG=chromium:809634
TEST=unit tests pass
CQ-DEPEND=CL:1060243

Change-Id: I99492246992f27a2fc792651dce0cbf965dbd0ff
Reviewed-on: https://chromium-review.googlesource.com/1045617
Commit-Ready: Benjamin Gordon <bmgordon@chromium.org>
Tested-by: Benjamin Gordon <bmgordon@chromium.org>
Reviewed-by: Benjamin Gordon <bmgordon@chromium.org>
diff --git a/cli/command_unittest.py b/cli/command_unittest.py
index 9c9e307..f228e11 100644
--- a/cli/command_unittest.py
+++ b/cli/command_unittest.py
@@ -14,7 +14,6 @@
 from chromite.lib import constants
 from chromite.cli import command
 from chromite.lib import commandline
-from chromite.lib import cros_build_lib_unittest
 from chromite.lib import cros_import
 from chromite.lib import cros_test_lib
 from chromite.lib import partial_mock
@@ -78,7 +77,7 @@
   def __init__(self, args, base_args=None):
     partial_mock.PartialMock.__init__(self)
     self.args = args
-    self.rc_mock = cros_build_lib_unittest.RunCommandMock()
+    self.rc_mock = cros_test_lib.RunCommandMock()
     self.rc_mock.SetDefaultCmdResult()
     parser = commandline.ArgumentParser(caching=True)
     subparsers = parser.add_subparsers()