pdtest: Exit non-zero for errors as well as PD output differences

pdtest: Exit non-zero for errors as well as PD output differences

Make the pdtest(1) executable return a non-zero exit code for errors
during test execution (as before) as well as for differences in the
decoder output (that's the change). Return either the number of errors
seen, or the number of tests which saw different output. Only return
zero in the absence of errors and differences.