Reuse C-Core's unit test logic in iOS test

Look at how we can have better reuse of existing test logic from c-core. In [Event Engine test](https://github.com/grpc/grpc/tree/master/test/core/event_engine), for example, one possibility is to expose its test util via test-only public headers and call from iOS test. Some of these might not be possible as they are based on gtest.

An existing similar example is our current [Cronet's end2end test setup ](https://github.com/grpc/grpc/blob/master/src/objective-c/tests/CronetTests/CoreCronetEnd2EndTests.mm)

Start by reading the Event Engine tests under test/core/event_engine and the existing setup in src/objective-c/tests/CronetTests/CoreCronetEnd2EndTests.mm. Determine which C-core test utilities can be exposed through test-only public headers and used by iOS tests despite gtest dependencies. Done means selected existing test logic is reused by the iOS test setup.

Written by the indexing model from the issue text.