Build on the test table in modify.js or modify_window.js to test the following. 

resumeFromCheckpoint=false

• change window interval
• change $source startAtOperation time to be further in the past

kafka testing

• basic scenario with real kafka $source
• topic change with resumeFromCheckpoint=false
• topic change with resumeFromCheckpoint=true (errors)

misc testing

• Go through engine validate modify path when there is no checkpoint to restore from (this happens when processor is in CREATED state, or when $source does not support checkpointing).
• use a test-only source

EDIT: Moved strikethrough items into SERVER-98373