iOS unit test for C-Core's libuv-based timer implementation

For verifying libuv timer's usability for iOS, add unit test coverage to [EventEngineTimerTests](https://github.com/grpc/grpc/blob/master/src/objective-c/tests/CoreTests/CoreTests/EventEngineTimerTests.mm) test suite. This can closely mirror existing c-core's test logic at

* https://github.com/grpc/grpc/blob/master/test/core/event_engine/test_suite/timer_test.cc

Note that in order to test libuv timer, it requires the [libuv-timer impl PR](https://github.com/grpc/grpc/pull/27513) to land and instantiation of grpc_event_engine::experimental::LibuvEventEngine as mentioned in [libuv_event_engine_test.cc](https://github.com/grpc/grpc/pull/27513/files#diff-18cf23f5d1a78c393d788d9ba5f15ba7f2e8df9abae2f8daa3b9e0d571496b42)