Add a simple proto_cc_library rule

It only supports a single .proto file without any dependencies, but
that's enough for now.

Change-Id: Ibd80c8594b90956c3c5b0b46db10cf991d7ed1ad
diff --git a/build_tests/BUILD b/build_tests/BUILD
index cce2e01..be6a201 100644
--- a/build_tests/BUILD
+++ b/build_tests/BUILD
@@ -1,5 +1,6 @@
 load('/tools/build_rules/ruby', 'ruby_binary')
 load('/aos/build/queues', 'queue_library')
+load('/tools/build_rules/protobuf', 'proto_cc_library')
 
 cc_test(
   name = 'gflags_build_test',
@@ -69,3 +70,20 @@
   ],
   size = 'small',
 )
+
+proto_cc_library(
+  name = 'proto_build_test_library',
+  src = 'proto.proto',
+)
+
+cc_test(
+  name = 'proto_build_test',
+  srcs = [
+    'proto.cc',
+  ],
+  deps = [
+    ':proto_build_test_library',
+    '//aos/testing:googletest',
+  ],
+  size = 'small',
+)