Update README.md: add test badge

Author: emicklei

README.md

@@ -1,5 +1,6 @@
 ## dot - little helper package in Go for the graphviz dot language
 
+[![Go](https://github.com/emicklei/dot/actions/workflows/go.yml/badge.svg)](https://github.com/emicklei/dot/actions/workflows/go.yml)
 [![Go Report Card](https://goreportcard.com/badge/github.com/emicklei/dot)](https://goreportcard.com/report/github.com/emicklei/dot)
 [![GoDoc](https://pkg.go.dev/badge/github.com/emicklei/dot)](https://pkg.go.dev/github.com/emicklei/dot)
 [![codecov](https://codecov.io/gh/emicklei/dot/branch/master/graph/badge.svg)](https://codecov.io/gh/emicklei/dot)