Cleanup repeated module mocking utils

- mock non existant module util was defined twice
- split it out properly from the rest of the utils, update all references
- this allows us to move this util out of the codebase along with other code, e.g. the image manipulation code

ghost/image-transform/test/transform.test.js

@@ -3,12 +3,12 @@ const sinon = require('sinon');
 const fs = require('fs-extra');
 const errors = require('@tryghost/errors');
 const manipulator = require('../../../../server/lib/image/manipulator');
-const testUtils = require('../../../utils');
+const mockUtils = require('../../../utils/mocks');
 
 describe('lib/image: manipulator', function () {
     afterEach(function () {
         sinon.restore();
-        testUtils.unmockNotExistingModule();
+        mockUtils.modules.unmockNonExistentModule();
     });
 
     describe('canTransformFileExtension', function () {
@@ -55,7 +55,7 @@ describe('lib/image: manipulator', function () {
                 return sharpInstance;
             });
 
-            testUtils.mockNotExistingModule('sharp', sharp);
+            mockUtils.modules.mockNonExistentModule('sharp', sharp);
         });
 
         it('resize image', function () {
@@ -118,7 +118,7 @@ describe('lib/image: manipulator', function () {
 
     describe('installation', function () {
         beforeEach(function () {
-            testUtils.mockNotExistingModule('sharp', new Error(), true);
+            mockUtils.modules.mockNonExistentModule('sharp', new Error(), true);
         });
 
         it('sharp was not installed', function () {

ghost/image-transform/test/utils/modules.js

@@ -0,0 +1,23 @@
+const Module = require('module');
+const originalRequireFn = Module.prototype.require;
+
+/**
+ * helper fn to mock non-existent modules
+ * mocks.modules.mockNonExistentModule(/pattern/, mockedModule)
+ */
+exports.mockNonExistentModule = (modulePath, module, error = false) => {
+    Module.prototype.require = function (path) {
+        if (path.match(modulePath)) {
+            if (error) {
+                throw module;
+            }
+            return module;
+        }
+
+        return originalRequireFn.apply(this, arguments);
+    };
+};
+
+exports.unmockNonExistentModule = () => {
+    Module.prototype.require = originalRequireFn;
+};