I just got the attached test failure on Emacs master under Ubuntu 18.04.5; it 
reports failures for mml-secure-en-decrypt-sign-1-3-double and for 
mml-secure-en-decrypt-sign-3.

The failure is not easily reproducible; when I run 'cd tests && make 
mml-sec-tests', the test succeeds. Perhaps mark these tests as unstable as well?