Remove dev dep imports in production code (#2937)

Fixes #2876 with @paulmelnikow's suggestion 

Moved imports of `ServiceTester` and `createServiceTester` to a separate file so that dev dependencies are not imported by service classes.

doc/deprecating-badges.md

@@ -43,7 +43,7 @@ Locate the test file(s) for the service, which can be found in `*.tester.js` fil
 With `DeprecatedService` classes we cannot use `createServiceTester()` so you will need to create the `ServiceTester` class directly. For example:
 
 ```js
-const { ServiceTester } = require('..')
+const { ServiceTester } = require('../tester')
 
 const t = (module.exports = new ServiceTester({
   id: 'imagelayers',
@@ -69,7 +69,7 @@ Here is an example of what the final result would look like for a test file:
 ```js
 'use strict'
 
-const { ServiceTester } = require('..')
+const { ServiceTester } = require('../tester')
 
 const t = (module.exports = new ServiceTester({
   id: 'imagelayers',

doc/rewriting-services.md

@@ -278,7 +278,7 @@ https://github.com/hapijs/joi/blob/master/API.md
 Switch to `createServiceTester`:
 
 ```js
-const t = (module.exports = require('..').createServiceTester())
+const t = (module.exports = require('../tester').createServiceTester())
 ```
 
 This may require updating the URLs, which will be relative to the service's base

doc/service-tests.md

@@ -37,14 +37,14 @@ We'll start by adding some boilerplate to our file:
 
 const Joi = require('joi')
 
-const t = (module.exports = require('..').createServiceTester())
+const t = (module.exports = require('../tester').createServiceTester())
 ```
 
 1. Import [Joi][] We'll use this to make assertions. This is the same library
    we use to define schema for validation in the main badge class.
 
 2. If our `.service.js` module exports a single class, we can
-   `require('..').createServiceTester()`, which uses convention to create a
+   `require('../tester').createServiceTester()`, which uses convention to create a
    `ServiceTester` object. Calling this inside
    `services/wercker/wercker.tester.js` will create a `ServiceTester` object
    configured for the service exported in `services/wercker/wercker.service.js`.