I think the public that uses those tools usually don't care about standard practices (yet?).

So congrats, you are on the forefront I guess.

Simulink has a concept called Test Harnesses which are models that isolate individual blocks for testing. The tests themselves are then driven programmatically from MATLAB

visually 🤭🤣

I was actually offered a Bachelor thesis topic by a company to write a test bench for a product in LabView.

From what they told me and my other engineering experience I'd suggest going with an approach similar to what's used with HDLs. For unit tests create test benches in the language itself which call the functions you want to test with a predefined input (e.g. from a file) and then analyse and save the output.

You can extend this to obtaining other information as well, but tbh I'll bet it's still gonna be a pain.

Hope that helps at least a little.

I tried MS Word a few times and decided that drag-and-drop UIs are not for me

Visual... programming languages? Yikes.