Add some simple unit tests for SettingsHandler

[nlebeck]

This is my first attempt at adding unit tests in the Dolphin codebase. It looks like most of the unit test files don't have many comments, but please let me know if test plan comments would be helpful here, or if you have any feedback on the scope or style of the tests.

[JosJuice]

Instead of having tests that both encrypt and decrypt and then check the result, I would suggest having tests that do just encryption and just decryption. (Possibly in addition to ones that do both. GetValueOnSameInstance can be kept the way it is, for instance.) SettingsHandler is implementing a format defined by Nintendo, and we have to make sure we read and write files in a way that's compatible with Nintendo's files.

Additionally, if you're able to, it would be nice to have tests for the edge case implemented in PR #8704.

[nlebeck]

Sounds good, I'll make that change!

Adding test coverage for the edge case in PR #8704 seems fun. I'll either do that in here or leave it for a follow-up PR depending on how tricky it is.

[nlebeck]

Changed (most of) the tests to test encryption and decryption separately. PTAL! I'm not used to writing tests where the input/output data is raw u8 arrays, so let me know if there are any style concerns.

I left a TODO for PR #8704. I'd like to add that coverage, but I have pretty limited free time these days, so it might be a while before I get to it.

[nlebeck]

Thanks for the review!

[JosJuice]

Please squash your commits into one.

[nlebeck]

Thanks for the review! Commits squashed.