sysinfotest: Move from custom dmidecode scripts to virCommandSetDryRun()

sysinfotest: Move from custom dmidecode scripts to virCommandSetDryRun()

Problem with custom dmidecode scripts is that they are hard to
modify, especially if we will want them to act differently based
on passed arguments. So far, we have two scripts which do no more
than 'cat $sysinfo' where $sysinfo is saved dmidecode output.

The virCommandSetDryRun() can be used to trick
virSysinfoReadDMI() thinking it executed real dmidecode.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>