Unify and minimize tester boilerplate (#2472)

I started using this one-line boilerplate a while back and it seems to tidy things up a bit.

services/gem/gem-downloads.tester.js

@@ -1,12 +1,9 @@
 'use strict'
 
 const Joi = require('joi')
-const createServiceTester = require('../create-service-tester')
-
 const { isMetric } = require('../test-validators')
 
-const t = createServiceTester()
-module.exports = t
+const t = (module.exports = require('../create-service-tester')())
 
 // total downloads
 t.create('total downloads (valid)')

services/gem/gem-owner.tester.js

@@ -1,10 +1,8 @@
 'use strict'
 
 const Joi = require('joi')
-const createServiceTester = require('../create-service-tester')
 
-const t = createServiceTester()
-module.exports = t
+const t = (module.exports = require('../create-service-tester')())
 
 t.create('users (valid)')
   .get('/raphink.json')

services/gem/gem-rank.tester.js

@@ -1,15 +1,13 @@
 'use strict'
 
 const Joi = require('joi')
-const createServiceTester = require('../create-service-tester')
 
 const isOrdinalNumber = Joi.string().regex(/^[1-9][0-9]+(ᵗʰ|ˢᵗ|ⁿᵈ|ʳᵈ)$/)
 const isOrdinalNumberDaily = Joi.string().regex(
   /^[1-9][0-9]+(ᵗʰ|ˢᵗ|ⁿᵈ|ʳᵈ) daily$/
 )
 
-const t = createServiceTester()
-module.exports = t
+const t = (module.exports = require('../create-service-tester')())
 
 t.create('total rank (valid)')
   .get('/rt/rspec-puppet-facts.json')

services/gem/gem-version.tester.js

@@ -1,12 +1,9 @@
 'use strict'
 
 const Joi = require('joi')
-const createServiceTester = require('../create-service-tester')
-
 const { isVPlusDottedVersionAtLeastOne } = require('../test-validators')
 
-const t = createServiceTester()
-module.exports = t
+const t = (module.exports = require('../create-service-tester')())
 
 t.create('version (valid)')
   .get('/formatador.json')