Simplify specs

[nikku]

Proposed Changes

Proposes to simplify and semantically group our specs. I hope this would allow us to easier test various cases, using the new npm run test:inspect tool.

Checklist

To ensure you provided everything we need to look at your PR:

• [x] Brief textual description of the changes present
• [ ] Visual demo attached
• [ ] Steps to try out present, i.e. using the @bpmn-io/sr tool
• [ ] Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}

[nikku]

@philippfromme As per our discussion I removed most of the huge specs, and replaced them with simpler variants.

It should now be easy to verify (visually) the outcome of layouting.

[philippfromme]

I rebased the branch.

[nikku]

:cool: