We need negative tests for a few string functions that are not in the dfdl spec. These are all variations of allowable functions but with a different number of arguments.

fn:string-length - with 0 arguments
fn:contains - with 3 arguments
fn:starts-with - with 3 arguments
fn:ends-with - with 3 arguments
fn:substring-before - with 3 arguments
fn:substring-after - with 3 arguments

These tests will have to be added after dpath is complete since these are valid functions in xpath.