Fix another failure in autotest rendertest

Summary:
EncryptedMessagePart::okDecryptMIME() checks the decryption result
error code (at messagepart.cpp line 1196), finds GPG_ERR_NO_SECKEY, and
causes the rendered HTML to contain the error message "No secret key".
I think that when inlinepgpencrypted-error.mbox.html was added, gpgme
returned a different error code.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D23017