Split up [github] testers (#2560)

- Update github contributors badge for `create-service-tester`.

services/github/github-contributors.tester.js

@@ -0,0 +1,21 @@
+'use strict'
+
+const Joi = require('joi')
+
+const t = (module.exports = require('../create-service-tester')())
+
+t.create('Contributors')
+  .get('/cdnjs/cdnjs.json')
+  .expectJSONTypes(
+    Joi.object().keys({
+      name: 'contributors',
+      value: Joi.string().regex(/^\w+$/),
+    })
+  )
+
+t.create('Contributors (repo not found)')
+  .get('/badges/helmets.json')
+  .expectJSON({
+    name: 'contributors',
+    value: 'repo not found',
+  })