Add test coverage for all diffing scenarios supported currently

[nithinssabu]

Proposed Changes

Related to https://github.com/camunda/web-modeler/issues/11218

• Adds test coverage for all diffing scenarios supported. See this document for the full list of supported cases.
• Use npm test to run all the tests.

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}