Unify scripts to test FFI c code

[OussamaSaoudi-db]

Please describe why this is necessary.

Currently, both the visit-expression and read-table ffi tests use two identical scripts for comparing test results. This is unnecessary code duplication.

Describe the functionality you are proposing.

Unify the scripts into a single one, and make both tests use that one.

Additional context

The testing code can be found in ffi/examples/read-table and ffi/examples/visit-expression.

The scripts can be found in ffi/tests/read-table-testing/run_test.sh and ffi/tests/test_expression_visitor/run_test.sh.