Added character comparison tests. I don't think these are entirely satisfactory; we should be able to test additional properties of the orderings.
Added tests for the twelve string comparison functions. string= has its own individual tests, but the others are only tested by calling a random test case generator 1000 times (as is string=).
Fixed problem string-capitalize.2; added tests for nstring-upcase, nstring-downcase, and nstring-capitalize.