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/tools/cpp/BUILD b/tools/cpp/BUILD
index af72b66..2721e32 100644
--- a/tools/cpp/BUILD
+++ b/tools/cpp/BUILD
@@ -68,3 +68,8 @@
     strip_files = ":empty",
     supports_param_files = 0,
 )
+
+filegroup(
+    name = "srcs",
+    srcs = glob(["**"]),
+)