NPAR TESTS: Consistently order variables in summary statistics.

The set of variables in the NPAR TESTS specs structure was ordered
randomly, according to however the hash function happened to arrange them.
Sort them by variable name, instead, so that they always appear in
alphabetical order in, e.g., descriptive statistics output.

The particular hash function PSPP uses now tends to order variables
alphabetically anyhow.  The next commit changes the PSPP hash functions,
so fixing this in advance prevents having to update any test output.