fix(api.js): Change cb type in mockIPC to return unknown (#11724)

Author: FabianLars

.changes/fix-apijs-mockipc-cb-type.md

@@ -0,0 +1,5 @@
+---
+"@tauri-apps/api": "patch:bug"
+---
+
+Removed the generic in the type of the callback function argument in `mockIPC` which prevented its proper use in tests using TypeScript.