As the certificate chain can be added after the document was
opened, we need to trigger signature verification every time the
LOK API method getSignatureState is called.

In addition update the tests so that they check the status of a
document that's not signed, a document that was signed but the
certificate chain is not available so the verification fails and
later adding the certificate chain and the verification returns
an OK status.

Change-Id: I44578d0cece5bfc4a2e43fbbcd68b5ea1ccbc38b
Reviewed-on: https://gerrit.libreoffice.org/62276
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>