[local_auth] Remove use of OCMock (#6757)

Adjusts the iOS implementation of local_auth to use a protocol wrapping the relevant parts of `LAContext` instead of using `LAContext` directly, allowing the tests to use a stub implementation instead of OCMock.

The goal is to unblock converting the unit tests to Swift, but even by itself this actually makes the tests much shorter and easier to understand.

Part of flutter/flutter#119104

Author: stuartmorgan-g

packages/local_auth/local_auth_darwin/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 1.3.1
+
+* Adjusts implementation for improved testability, and removes use of OCMock.
+
 ## 1.3.0
 
 * Adds Swift Package Manager compatibility.