bug#44259: Most mml-sec.el tests are failing on macOS

[Stefan Kangas]

Most mml-sec.el tests are failing on macOS.  Here is my analysis.

In mml-sec-tests.el, the cleanup in mml-sec-test--kill-gpg-agent
depends on the 'args key in the alist returned by process-attributes.
But evaluating following on macOS gives nil:

   (assq 'args (process-attributes (emacs-pid)))

Support for `args' seems to be not implemented on macOS.  See
src/sysdep.c, function system_process_attributes.

This was previously discussed in Bug#16579, where support for
process-attributes was first added. It seems like the API for all this
is not documented, so it's tricky to add.

    https://debbugs.gnu.org/cgi/bugreport.cgi?bug=16579#5

So I guess we either should make the tests not depend on 'args, or
figure out how to implement the above support for macOS.