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/lib.go

@@ -0,0 +1,11 @@
+package somelib
+
+import "fmt"
+
+func Name() string {
+	return "edef"
+}
+
+func Greet(s string) string {
+	return fmt.Sprintf("Hello %s", s)
+}