test: verify nested change with context

[nikku]

Proposed Changes

Adds a test case to be used as a foundation for work on #20.

Checklist

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• 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