DEV: Resolve and prevent user fixture changes leaking between tests (#23898)

- Introduces a `deepFreeze` helper to block any mutations to the current-user fixture

- Add `cloneJSON` to any places which were previously causing mutations

app/assets/javascripts/discourse-common/addon/lib/object.js

@@ -59,3 +59,19 @@ export function deepEqual(obj1, obj2) {
 export function cloneJSON(obj) {
   return JSON.parse(JSON.stringify(obj));
 }
+
+export function deepFreeze(object) {
+  // Retrieve the property names defined on object
+  const propNames = Reflect.ownKeys(object);
+
+  // Freeze properties before freezing self
+  for (const name of propNames) {
+    const value = object[name];
+
+    if ((value && typeof value === "object") || typeof value === "function") {
+      deepFreeze(value);
+    }
+  }
+
+  return Object.freeze(object);
+}