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.
Actually the ```std::string``` just to concatenate using ```+``` is totally unnecessary. That line should just be
```
CPPUNIT_ASSERT( std::string(if1) == "if" "(x<2*y,x*x+y,y*y+x)" );
```
Adjacent string literals separated by whitespace are automatically concatenated in C/C++ .
And that wouldn't look too crazy in my opinion.