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

[dennycd]

For verifying libuv timer's usability for iOS, add unit test coverage to EventEngineTimerTests 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 to land and instantiation of grpc_event_engine::experimental::LibuvEventEngine as mentioned in libuv_event_engine_test.cc

[dennycd]

initial libuv setup PR landed. This one is good to go