BuildAPI: Add unit tests and mocks for Dependency Service.

BUG=chromium:1000851
TEST=run_tests

Change-Id: Ib984eaa6e5dd812094bd27f103eaa70cf375d6ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1922991
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Michael Mortensen <mmortensen@google.com>
Tested-by: Michael Mortensen <mmortensen@google.com>
diff --git a/api/controller/dependency.py b/api/controller/dependency.py
index af7a453..072175d 100644
--- a/api/controller/dependency.py
+++ b/api/controller/dependency.py
@@ -49,7 +49,13 @@
       source_path.path = path
 
 
-@faux.all_empty
+def _GetBuildDependencyGraphResponse(_input_proto, output_proto, _config):
+  """Add fake dep_graph data to a successful response."""
+  output_proto.dep_graph.build_target.name = 'target_board'
+
+
+@faux.success(_GetBuildDependencyGraphResponse)
+@faux.empty_error
 @validate.require('build_target.name')
 @validate.validation_complete
 def GetBuildDependencyGraph(input_proto, output_proto, _config):