tests: Fix generation of keys without expiration

* tests/run-genkey.c (parse_expire_string): Support default expiration
and no expiration. Set flag for no expiration.
(main): Allow update of flags when generating new key.

This makes the parsing of different values for expiration match the
parsing done by gpg's --quick-gen-key. In particular, this makes it
possible again to generate keys without expiration.