[tests] refactor tests to simplify future changes

tests/constant_propagation/array_pointers.i

tests/constant_propagation/bts-1787-bis.i

tests/constant_propagation/bts-1787.i

tests/constant_propagation/const_field_return_struct.i

tests/constant_propagation/enum.i

tests/constant_propagation/fct_ptr.i

tests/constant_propagation/struct_field.i

tests/constant_propagation/test_config