feat(tools): Add 'gotest' program to demonstrate pkgs.buildGo

This is a tiny program that does nothing but exists to demonstrate
pkgs.buildGo by building a program that depends on a local library as
well as a protobuf definition.

tools/gotest/main.go

@@ -0,0 +1,16 @@
+// This program just exists to import some libraries and demonstrate
+// that the build works, it doesn't do anything useful.
+package main
+
+import (
+	"fmt"
+	"somelib"
+	"someproto"
+)
+
+func main() {
+	p := someproto.Person{
+		Name: somelib.Name(),
+	}
+	fmt.Println(somelib.Greet(fmt.Sprintf("%v", p)))
+}