Not all channels used for credential transport in systemd are trusted
on SNP: while cmdline and initrd are measured and should be used,
smbios and fwcfg are untrusted and the guest should ignore creds from
those delivery paths.
Implement an offensive credential test that tries to inject credentials
via those untrusted channels and check the guest won't consume those
credentials.
Signed-off-by: Paul Meyer <katexochen0@gmail.com>