Unit testing completion blocks in iOS

I am trying to figure out how to write unit test for client backend code but I have not been successful so far. I am not so experience with unit testing so I read This, This, andThis

and read some similar questions to what I am asking, but I have not been completely figure out how I should be writing unit tests for the client backend.

- (void)testThatItGetsAllUsers
{
    // given
    XCTestExpectation *expectation = [self expectationWithDescription:@" fetch all users"];

    // when
    [[Backend sharedInstance] getAllUsers:^(NSArray * arr) {

            XCTAssertNotNil(arr);
            [expectation fulfill];

    }andFailure:^(NSError * err) {
        XCTAssert((err != nil), @"get all users request failed with error:\t%@", err);

    }];

    [self waitForExpectationsWithTimeout:5 handler:^(NSError *error) {
     // request not successfull
        XCTAssert((error != nil), @"get all users from server did not return any data:\t%@", error);
    }];
    // then
}

It would be really helpful if someone can point out what I am doing wrong and any additional knowledge is appreciated!