Update or deprecate old tests for `addStage()`

[laacri]

• The tests using test_patients_staging_uicc7.json, test_patients_staging_uicc8.json and test_patients_staging_general.json are failing as they relied on a previous version of addStages(). Either update the expectations using the new function or send the old tests to a extras/deprecated.R file and comment them

• Check if the last test ("Overlap of TNM codes") is still relevant or is superseded in test-saveTNMRules.R