Expand unit tests for built-in functions

[simicd]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Hi @andygrove,

First of all I wanted to thank you for creating and open-sourcing datafusion - I have been following the project for some time and find it super exciting to see the next-gen data engineering infrastructure being built on top of Arrow!

I have no experience with Rust but quite some with writing applications in Python & PySpark so I thought I could contribute to the Python language bindings.

Recently I saw @francis-du added lots of functions to the Python package with #73 - thanks for the significant effort in improving the package!

To improve the test coverage, I've opened pull request #129 with a few unit tests for the built-in functions. Hope the tests are helpful, look forward to getting some feedback.

Describe the solution you'd like
Improve test coverage by adding more unit tests.

Describe alternatives you've considered
n/a

Additional context
n/a