Add a build rule for the boilerplate unit test.

hack/boilerplate/BUILD

@@ -1,3 +1,12 @@
 package(default_visibility = ["//visibility:public"])
 
 exports_files(glob(["*.txt"]))
+
+py_test(
+    name = "boilerplate_test",
+    srcs = [
+        "boilerplate_test.py",
+        "boilerplate.py",
+    ],
+    data = glob(["*.txt", "test/*"]),
+)