It should not be possible to create an async verifier for a regular subject

[sharplet]

This spec fails, and it's kind of confusing:

it(@"should not allow regular subjects to be attached to async verifiers", ^{
    __block BOOL called = NO;

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        [NSThread sleepForTimeInterval:0.1];
        called = YES;
    });

    [[theValue(called) shouldEventually] beYes];
});

The problem is very non-obvious: we should be using expectFutureValue() so that we can poll until the value changes.

This should fail with a more descriptive error, pointing users to expectFutureValue(). Ideally, it would be great if there was a compiler warning or error to prevent this test from ever running.