Add a shallowSnapshot option for storyshots test functions

[heinzmuller]

Issue: #995 StoryShots doesn't have a preset for shallow rendering snapshots

What I did

Added a test body for shallow rendering snapshots

How to test

Add this as a storyshot test file

import initStoryshots, { shallowSnapshot } from '@storybook/addon-storyshots'

initStoryshots({
  test: shallowSnapshot
})

[codecov]

Codecov Report

Merging #1232 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1232      +/-   ##
==========================================
- Coverage   13.73%   13.71%   -0.03%     
==========================================
  Files         207      207              
  Lines        4638     4646       +8     
  Branches      510      512       +2     
==========================================
  Hits          637      637              
- Misses       3550     3565      +15     
+ Partials      451      444       -7

Impacted Files	Coverage Δ
addons/storyshots/src/test-bodies.js	0% <0%> (ø)	⬆️
addons/storyshots/src/index.js	0% <0%> (ø)	⬆️
...rc/modules/ui/components/left_panel/text_filter.js	33.33% <0%> (ø)	⬆️
addons/info/src/components/markdown/code.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/handle_routing.js	28.94% <0%> (ø)	⬆️
app/react/src/client/preview/client_api.js	39.28% <0%> (ø)	⬆️
addons/knobs/src/components/PropForm.js	8.51% <0%> (ø)	⬆️
...codemod/src/transforms/update-organisation-name.js	40.62% <0%> (ø)	⬆️
addons/info/src/components/PropVal.js	0% <0%> (ø)	⬆️
... and 18 more

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[ndelangen]

Can you explain why you'd want to shallow render stories? Is this something people want to do?

[heinzmuller]

I can only speak for myself, but when you have huge library of smaller react components, if you don't do shallow rendering you will get huge snapshot mismatches when you change those components. Granted that snapshot tests are really easy to update, but when you have large mismatches it is also increases the likelihood that you miss a real error.

And secondly when you shallow render, it makes it that more important to make sure you have tests for all components.

PS. It's not important for me to have this functionality built into storybooks as it's really easy to do yourself, but I was asked in #995 to make a PR. If this isn't something you'd like to have added that is fine aswell! :)

[ndelangen]

@tmeasday Can you review this?

[tmeasday]

Works great, Thanks @heinzmuller !