Improve unit test coverage for complexify fortran library

# Description
Currently, parts of the [complexify fortran library](https://github.com/mdolab/complexify/blob/main/src/complexify.f90) are not unit tested. We need to continue and add more [unittests](https://github.com/mdolab/complexify/blob/main/tests/test_complexify.f90) as we should have full coverage for the library. 