add unit test

test/BUILD

@@ -1,5 +1,5 @@
 load("//rules:rules_go.bzl", "go_binary")
-load("@io_bazel_rules_go//go:def.bzl", "go_library")
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
 
 go_library(
     name = "hello_lib",
@@ -16,3 +16,9 @@ go_binary(
     static = "on",
     visibility = ["//visibility:public"],
 )
+
+go_test(
+    name = "test_test",
+    srcs = ["hello_test.go"],
+    embed = [":test_lib"],
+)

test/hello.go

@@ -5,9 +5,14 @@ import (
 )
 
 // #include <stdio.h>
-// void helloworld() { printf("hello, world\n"); }
+// char* hello() { return "hello, world\n"; }
+// void printhello() { printf(hello()); }
 import "C"
 
 func main() {
-	C.helloworld()
+	C.printhello()
+}
+
+func chello() string {
+	return C.GoString(C.hello())
 }