* Prerequisites
I tested against Emacs versions 24.3 (precompiled on my system, with Gnus
v5.13) and 25.0.50 (compiled from git source, with the included v5.13 and with
Ma Gnus v0.14).
In general, I recommend that you use GnuPG version 1.x for tests.  Obviously,
for gpgsm you need 2.x, which works for me with 2.0.x but not 2.1.x; see
mml-secure-run-tests-with-gpg2 in gnustest-mml-sec.el.  When running tests
with different versions of GnuPG make sure that proper versions of gpg-agent
are running (kill all prior to testing, if in doubt).


* Test keys
The subdirectory mml-gpghome contains OpenPGP and S/MIME test keyrings for
GnuPG’s gpg and gpgsm commands.  In addition, it contains a file
gpg-agent.conf where all options are commented out.  In particular, by
activating the debug settings one can verify whether the correct version of
gpg-agent is running and whether pinentry problems arise with the current
setup.

Most keys in the test keyrings come with empty passphrases, while the keys
associated with the user ID “No Expiry two UIDs” have the passphrase
“Passphrase”.  You can see all public keys and user IDs as follows:
$ gpg --homedir ./mml-gpghome --fingerprint -k --list-options show-unusable-subkeys,show-unusable-uids
$ gpgsm --homedir ./mml-gpghome -k


* Running tests
To run all tests:
$ cd <path-to-gnus/lisp/test>
$ emacs -Q -batch -L .. -l gnustest-mml-sec.el -f mml-secure-run-tests

However, in the above case gpgsm will ask for passphrases, even empty ones.
To omit those tests:
$ emacs -Q -batch -L .. -l gnustest-mml-sec.el -f mml-secure-run-tests-without-smime

To run all tests with epg-gpg-program set to "gpg2":
$ emacs -Q -batch -L .. -l gnustest-mml-sec.el -f mml-secure-run-tests-with-gpg2

To check an issue with truncation of y-or-n-p questions:
$ emacs -Q -L .. -l gnustest-mml-sec.el -f mml-secure-select-preferred-keys-todo
Then mark one or two keys and select “OK”.  The following question should be
truncated.  Answer “n” to avoid storage of that choice in your ~/.emacs.

To see that question entirely (outside an encryption context):
$ emacs -Q -L .. -l gnustest-mml-sec.el -f mml-secure-select-preferred-keys-ok