[TEST] Low coverage on utility_functions.py

[genedan]

Description

I'm working on this. Most likely this will require 2 or 3 PRs since the missing lines cover a variety of different functions (I/O, patsy, model diagnostics).

Is your feature request aligned with the scope of the package?

• Yes, absolutely!
• No, but it's still worth discussing.
• N/A (this request is not a codebase enhancement).

Describe the solution you'd like, or your current workaround.

No response

Do you have any additional supporting notes?

No response