Open-source Bazel integration tests

It is a first batch of integration tests for Bazel. They tests some
basic behaviors and nominal cases of Bazel rules (especially they tests
the behavior of the examples provided in //examples).

--
MOS_MIGRATED_REVID=89440074
diff --git a/src/main/protobuf/BUILD b/src/main/protobuf/BUILD
index 973bf74..717d9bf 100644
--- a/src/main/protobuf/BUILD
+++ b/src/main/protobuf/BUILD
@@ -14,3 +14,8 @@
     "bundlemerge",
     "xcodegen",
 ]]
+
+filegroup(
+    name = "srcs",
+    srcs = glob(["**"]),
+)