public class PostsToChatter {
    public interface ChatterPoster {
        void postToChatter(Id parentRecord, String text);
    }

    @TestVisible private class RealChatterPoster implements ChatterPoster {
        public void postToChatter(Id parentRecord, String text) {
            if (!Test.isRunningTest()) callConnectAPIHere(parentRecord, text);
            // Note that if we keep this minimal (one line) we still obtain full code coverage even though the Connect API call does not actually go through.
        }
    }

    @TestVisible private ChatterPoster myPoster;


    // ... constructors and code ...

    public PostsToChatter() {
        // Add a default delegate (the real Chatter poster class
        myPoster = new RealChatterPoster();
    }

    public void doSomethingRequiringAChatterPost() {
        // ... stuff happens ...
        myPoster.postToChatter(someId, someText);
    }
}

@isTest
public static class PostsToChatter_TEST {
    private class MockChatterPoster implements PostsToChatter.ChatterPoster {
        public Id expectedId;
        public String expectedString;
        public Integer timesCalled = 0;

        public void postToChatter(Id parentRecord, String text) {
            System.assertEquals(expectedId, parentRecord, 'correct parent');
            System.assertEquals(expectedString, text, 'correct post body');
            timesCalled++;
        }
    }

    @isTest
    public static void testTheClass() {
        PostsToChatter p = new PostsToChatter();
        p.myPoster = new MockChatterPoster();

        // ... invoke class functionality...

        System.assertEquals(1, p.myPoster.timesCalled, 'chatter post called');
        // Post contents validated in the mock - or do it here if preferred.
    }
}