This also switches around the test for the self signed key to now fail, as the
generated certificate is no longer loaded into the trust list. This is a more
useful test as it more accurately represents what the fwupd daemon is doing.
As a side-note the detached signature from the derivate cannot be generated
using `--no-p7-include-cert` as only the main LVFS-CA certificate is shipped
with fwupd.
We can use this as an alternative for GPG. No PKCS7 certificates are currently
installed by fwupd and it's expected that the LVFS will still only provide GPG
detached signatures.
If an OEM distributor wants to sign firmware with a PKCS7 and the corresponding
certificate is provided then the firmware will be marked as valid.
Only firmware shipping with a .p7b file will use the PKCS7 functionality,
similarly remote metadata validation will default to GPG unless Keyring=pkcs7
is specified in the config file.
