Allow custom test function for storyshots

[tmeasday]

Currently, storyshots does two things: (a) it sets up a Jest test for each story, and (b) it defines the body of that test to be a vanilla snapshot test.

However, there are different things you may want to do in step (b), for instance:

• Simply render the story, and ensure it doesn't throw an exception (Add a noMatchSnapshots option. #917, Support for testing storybook breaks on each code commit #547, Reflect render errors on left panel #1032)
• Shallow render the story, and snapshot it (Option to enable shallow rendering #995)
• Pass options to the rendering, especially createNodeMock (TypeError: w.default.findDOMNode is not a function #876, Allow passing options to react-test-renderer .create() method #881, Pass options to test renderer storybook-eol/storyshots#98)

This issue proposes allowing configuring the body of the test (although keeping the current as the default). First proposed here: #917 (comment)

There is a question of how to best implement this. The simplest thing is simply an option to initStoryshots:

initStoryshots({
  test(story) {
    // make assertions (doesn't return anything).
  }
});

@theinterned made a suggestion around lifecycle methods here -- can you provide a bit more detail about how that might be used?

In terms of implementing the use cases above, I would suggest we export "test bodies" that do the standard things from storyshots. So you could do

import initStoryshots, { shallowSnapshot } from 'storyshots';

initStoryshots({ test: shallowSnapshot });

[tmeasday]

I implemented my suggestion in #1035, but we can do something else still.

[RohovDmytro]

There is a question of how to best implement this.

@tmeasday maybe extract step b into an own package? And storyshots addon can be used on top of it. Other addons can be build too (like #1032 or else)

Thus storyshots can still be focused on jest testing.

[tmeasday]

@rogovdm do you mean to extract this file into its own package?

[ndelangen]

Is this done ? @tmeasday

[tmeasday]

Yes, that is what I meant by the merged label @ndelangen -- I thought that was the agreed process?

[tmeasday]

Released with v3!