Allow passing a test function to storyshots

See #1034

addons/storyshots/src/index.js

@@ -1,11 +1,13 @@
-import renderer from 'react-test-renderer';
 import path from 'path';
 import readPkgUp from 'read-pkg-up';
 import addons from '@storybook/addons';
 import runWithRequireContext from './require_context';
 import createChannel from './storybook-channel-mock';
+import { snapshot } from './test-bodies';
 const { describe, it, expect } = global;
 
+export { snapshot, renderOnly } from './test-bodies';
+
 let storybook;
 let configPath;
 
@@ -59,6 +61,8 @@ export default function testStorySnapshots(options = {}) {
   // Added not to break existing storyshots configs (can be removed in a future major release)
   options.storyNameRegex = options.storyNameRegex || options.storyRegex;
 
+  options.test = options.test || snapshot;
+
   for (const group of stories) {
     if (options.storyKindRegex && !group.kind.match(options.storyKindRegex)) {
       continue;
@@ -73,9 +77,7 @@ export default function testStorySnapshots(options = {}) {
 
           it(story.name, () => {
             const context = { kind: group.kind, story: story.name };
-            const renderedStory = story.render(context);
-            const tree = renderer.create(renderedStory).toJSON();
-            expect(tree).toMatchSnapshot();
+            options.test({ story, context });
           });
         }
       });

addons/storyshots/src/test-bodies.js

@@ -0,0 +1,13 @@
+import renderer from 'react-test-renderer';
+import shallow from 'react-test-renderer/shallow';
+
+export function snapshot({ story, context }) {
+  const storyElement = story.render(context);
+  const tree = renderer.create(storyElement).toJSON();
+  expect(tree).toMatchSnapshot();
+}
+
+export function renderOnly({ story, context }) {
+  const storyElement = story.render(context);
+  const tree = renderer.create(storyElement);
+}