How to Fix 'Field is Not Writable' Error (Easily)

[@TimPaduraru]

I think they changed the JSON.deserialize function since then. the new sytanx for API 60:
Case mockCase = (Case)JSON.deserialize('{....}', Case.class);

[@KseniyaTsimashenka]

I've got this error: "Variable does not exist: TestRecordUtility"