Chore/Add additional FFI unit tests (#14802)

* Add unit tests to FFI_ExecutionPlan

* Add unit tests for FFI table source

* Add round trip tests for volatility

* Add unit tests for FFI insert op

* Simplify string generation in unit test

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

* Fix drop of borrowed value

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Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

Author: timsaucer