test: add snapshot tests for MessageBuilder

[Nerixyz]

I want to refactor the MessageBuilder for IRC messages, but it's an integral part of Chatterino, so I'm adding some tests first. These test that the output message is the same. Adding tests requires the following steps (also documented in the MessageBuilder test (at the top).

1. Add an IRC message in tests/snapshots/MessageBuilder/IRC as a new JSON file ({ "input": "<IRC-msg>" }) and save it
2. Set UPDATE_SNAPSHOTS in tests/src/MessageBuilder.cpp to true
3. In the same file, add the filename from step 1 without the extension to IRC_SNAPSHOTS
4. Run the new test (or run all message-builder tests with ctest -R TestMessageBuilderP --output-on-failure). Don't worry about the integrity test for now.
5. Set UPDATE_SNAPSHOTS back to false
6. Ensure the output is stable by running the message-builder tests again ctest -R TestMessageBuilderP --output-on-failure (everything should pass now).

On Windows, you might see that all snapshot files will show as modified. However, after staging them (git add), they shouldn't show up as modified anymore (as just the line-endings changed).

Coverage from this PR: https://app.codecov.io/github/Chatterino/chatterino2/pull/5598

[github-actions]

clang-tidy made some suggestions

[pajlada]

Partial review, haven't taken a full look
Overall looks good and will definitely bring confidence in changing the MessageBuilder logic going forward