benchmark-unify: map krb5-23 format to NT

Since krb5-23 is the same as NT, krb5-23 hat been removed.
The other implementations (nt and nt2) had been much faster anyway.

That's why, benchmark-unify maps krb5-23 to nt, to avoid relbench's
"Only in file 1: " or "Only in file 2: " output, and possibly even the
"Converting the two benchmark files using benchmark-unify might\n"
"increase the number of benchmarks which can be compared\n"
output in relbench.