Unit testing is quite useful in the right circumstances, especially to automate testing of certain functions.
Such as testing allowed address ranges, to make sure the right function is achieved.

The goal is not to unit test every function of firmware, but a selected portion to make sure the intended outcome of that function is not broken and being able to test edge-cases and so forth.

For our use case there is multiple openings, but being able to easily mock functions/hardware dependencies is required.
Potentially the tests can be run in QEMU, to run on the right architecture, but it is at least desirable that it can run in CI.

A short list of requirements: open source; maintained; mocking.

Possible frameworks to use is:
Cmocka - https://cmocka.org/
Unity - https://www.throwtheswitch.org/unity (and its moca)
GoogleTest - https://github.com/google/googletest
Check - https://libcheck.github.io/check/

The list above is sorted in order of which seems most interesting at the top.