Add unit tests for Utils::toHex

[mtlynch]

No description provided.

[mtlynch]

I know toHex isn't the most interesting function to test, but this is just a proof of concept of the idea of unit testing in this project.

Unit testing can make it significantly easier and faster to catch bugs, as we can exercise logic on regular desktops and in Github Actions without having to push code to a LoRa device and test it. It also allows maintainers to see immediately in Github Actions if a change caused a regression in tests.

I chose toHex just because it's a simple function with no external dependencies, though because it's defined in a file that also includes dependencies on Stream.h and SHA256.h, we need to mock out those dependencies.