[chore] Remove omitempty on account source; refactor tests to use prettyprint json

[tsmethurst]

Description

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This pull request updates some of the account serialization logic to remove some omitEmpty annotations that were causing trouble + resulting in missing fields. Tests have been updated for this.

While I was in the code, I also refactored tests elsewhere in the codebase to use prettyprinted json when non-trivial json is being diffed. This lets us more easily see what's failing when tests fail. The actual substance of the tests has not changed at all, it's purely formatting.

closes #1332

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.