cpp,tests: Launch dirmngr with gpg-conf

* lang/cpp/tests/run-wkdlookup.cpp (main): Replace backslashes with
forward slashes in homedir returned by GpgME::dirInfo. Use "gpgconf
--launch dirmngr" to start dirmngr.

With this change the same code as by QGpgMEWKDLookupJob is again used to
launch dirmngr. It also fixes the bad example of starting dirmngr
without the help of gpgconf.

• GnuPG-bug-id: T6833