Introduce snapshot testing

[philippfromme]

Proposed Changes

Allows us to easily compare before and after for the defined test cases. Could be added to the CI so the CI only passes once all snapshots pass meaning you either haven't changed the algorithm or you did and you updated the snapshots.

• snapshot tests are part of npm test and a snapshot that is not matching will result in an unsuccessful test run
• snapshots that are matching are highlighted in green
• snapshots that are not matching are highlighted in red
• if no snapshot exists no highlight is added

### Checklist To ensure you provided everything we need to look at your PR: * [x] **Brief textual description** of the changes present * [x] **Visual demo** attached * [ ] **Steps to try out** present, i.e. [using the `@bpmn-io/sr` tool](https://github.com/bpmn-io/sr) * [ ] Related issue linked via `Closes {LINK_TO_ISSUE}` or `Related to {LINK_TO_ISSUE}`

[nikku]

Let's get this incorporated soonish, in order to test the other open PRs.

[philippfromme]

I'll get this PR ready.

[barmac]

@philippfromme looks that we need to make sure our snapshots are initially valid.

[philippfromme]

@barmac Thought I'd done that 🙈

[abdul99ahad]

Some of the test cases are not showing in the left panel.

[philippfromme]

Some of the test cases are not showing in the left panel.

This is because the original diagram doesn't contain any graphical information. While the use case of the library is to create missing graphical information for the snapshot tests it's actually more convinient to have a diagram with graphical information. The library will then simply replace that. I'll update the diagrams that have no DI.

[philippfromme]

I updated the diagrams without DI and also added an error indication if the import fails completely. Should be clearer now.

[philippfromme]

Ready for review.

[abdul99ahad]

Ready for review.

Looks good to me 🚀

[nikku]

Updated with a minor hint regarding test updates: