Split up [github] testers (#2560)

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

services/github/github-followers.tester.js

@@ -0,0 +1,21 @@
+'use strict'
+
+const Joi = require('joi')
+
+const t = (module.exports = require('../create-service-tester')())
+
+t.create('Followers')
+  .get('/webcaetano.json')
+  .expectJSONTypes(
+    Joi.object().keys({
+      name: 'followers',
+      value: Joi.string().regex(/^\w+$/),
+    })
+  )
+
+t.create('Followers (user not found)')
+  .get('/PyvesB2.json')
+  .expectJSON({
+    name: 'followers',
+    value: 'user not found',
+  })