test: verify nested change with context

[nikku]

Proposed Changes

Adds a test case to be used as a foundation for work on https://github.com/bpmn-io/bpmn-js-differ/issues/20.

Checklist

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

• [ ] 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]

CC @nithinssabu