Moved tests from JdbcScalarFunctionTest to RelationalScalarFunctionTest

I didn't see why, so I tried to implement it. Turns out your substring implementation was wrong in the first place. You should have had some `end-start` in there somewhere, because Scala substring takes start position and end position, but SQL substring take start position and length. I pointed that out in some comment to a previous PR, but maybe I wasn't clear enough.