tests: Fix conversion of random zbase32 data to string

* tests/run-genrandom.cpp (main): Pass data to std::string constructor
instead of copying it byte by byte.

The byte-by-byte conversion erroneously copied the terminating null byte
into the string. Passing the random \0-terminated zbase32 data to the
std::string constructor fixes the conversion and demonstrates how I
envisioned the conversion to a string. Unfortunately, one has to
reinterpret the pointer.

• GnuPG-bug-id: T8108