tests: Add two more multipart-signed tests
* tests/data/openpgp-signed-no-attach-gpgol.mbox, tests/data/openpgp-signed-no-attach-gpgol.plain, tests/data/openpgp-signed-two-attachments.mbox, tests/data/openpgp-signed-two-attachments.plain: New. * tests/t-parser.cpp: Use them.
The attachment test includes inline attachments.
The gpgol test adds a message like it is seen by gpgol
(Without headers)