Rename and fix if/conditional in ExpressionBuilder 

This is a minor change that renames and fixes a previously badly named and _broken_ feature of ```ExpressionBuilder```. Closes #4470.
@aeslaughter means the ```"if(a,b,c)"``` FParser expression (inside a string). There really is no straight forward way to allow this in the test. The expression could have been ```"5+if(a,b,c)"``` so allowing ```"if(``` as we discussed earlier is not a solution either. 

I think obfuscating like I did here is the easiest way around. There are other methods than using ```std::string``` concatenation, like macros or multiline strings. Let me know if you guys think of a better way.