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 <[email protected]>

* Fix drop of borrowed value

---------

Co-authored-by: Andrew Lamb <[email protected]>

Author: timsaucer