Since rpm-software-management#1679
`sdbus::ObjectPath` is send over to the client not std::string.

This was causing the check `signature_valid(sdbus::Signal &)` to crash,
the client callback ended and the server kept waiting for result
of the `key_import` userconfirm.

A serialized offline transaction consists only of local packages
downloaded to the `<system state dir>/offline` location. Unfortunately,
PGP checks for these packages are disabled by default due to the
`localpkg_gpgcheck` option default value.
Instead of testing the serialized transaction, this patch performs an
RPM transaction test on the originally resolved transaction, which still
retains information about the repositories from which the packages were
downloaded.
This approach also saves time required for deserialization and resolving
the testing transaction.

This ports the fix from rpm-software-management#1672
to dnf5daemon.