Tests/logic check text

What this does
---

- Informs logs if you try to add a KifTestStep by array, and it is one kif test step. Makes debugging easier, rather than a random unknown selector error.

- If the check text in stepToCheckText is nil, return success, as there is nothing to check. We handle nil here, as it would be ugly to do so in code. Everything would be : 
```Objective-C
    if (text) {
     [steps addStep:[KIFTestStep stepToCheckText:text inViewWithAccessibilityLabelLike:@"View"]];
    }
```
  
rather than the step itself