tests: Add symmetric decryption tests.
* tests/openpgp/decrypt-sym.scm: New. * tests/openpgp/samplemsgs/enc-sym-cfb-1.asc: New. * tests/openpgp/samplemsgs/enc-sym-cfb-2.asc: New. * tests/openpgp/samplemsgs/enc-sym-ocb-1.asc: New. * tests/openpgp/samplemsgs/enc-sym-ocb-2.asc: New.
It's time to have some OCB tests in our suite so that we don't forget
to run the tests from our RNP interop tests. Also adds new tests for
CFB messages.