bug#42720: 28.0.50; mml-first-secure-en-decrypt-sign-1 test failing on Fedora 31

[Paul Eggert <eggert@cs.ucla.edu>]

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"make -j5 check" just failed for me on Ubuntu 18.04.5 with similar symptoms in 
mml-secure-en-decrypt-sign-2 so I took the liberty of marking that unstable too. 
Failing log attached, not that the log helps much.

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Running 19 tests (2020-08-12 18:31:44-0700, selector `(not (or (tag :expensive-test) (tag :unstable)))')
   passed   1/19  mml-secure-en-decrypt-1 (0.459199 sec)
   passed   2/19  mml-secure-en-decrypt-2 (0.470843 sec)
   passed   3/19  mml-secure-en-decrypt-3 (0.498642 sec)
   passed   4/19  mml-secure-en-decrypt-4 (0.456075 sec)
  skipped   5/19  mml-secure-en-decrypt-passphrase-cache (0.001965 sec)
  skipped   6/19  mml-secure-en-decrypt-passphrase-no-cache-openpgp-todo (0.003543 sec)
   failed   7/19  mml-secure-en-decrypt-passphrase-no-cache-smime-todo (0.001812 sec)
   passed   8/19  mml-secure-en-decrypt-sign-1-2-double (0.572457 sec)
Test mml-secure-en-decrypt-sign-2 backtrace:
  signal(epg-error ("Encrypt failed" "Exit"))
  mml-secure-test-fixture(#f(compiled-function () #<bytecode 0x6cd2142
  mml-secure-test-key-fixture(#f(compiled-function () #<bytecode -0x27
  #f(compiled-function () #<bytecode -0x191bb3e9cac0d201>)()
  ert--run-test-internal(#s(ert--test-execution-info :test #s(ert-test
  ert-run-test(#s(ert-test :name mml-secure-en-decrypt-sign-2 :documen
  ert-run-or-rerun-test(#s(ert--stats :selector ... :tests ... :test-m
  ert-run-tests((not (or (tag :expensive-test) (tag :unstable))) #f(co
  ert-run-tests-batch((not (or (tag :expensive-test) (tag :unstable)))
  ert-run-tests-batch-and-exit((not (or (tag :expensive-test) (tag :un
  eval((ert-run-tests-batch-and-exit '(not (or (tag :expensive-test) (
  command-line-1(("-L" ":." "-l" "ert" "-l" "lisp/gnus/mml-sec-tests" 
  command-line()
  normal-top-level()
Test mml-secure-en-decrypt-sign-2 condition:
    (epg-error "Encrypt failed" "Exit")
   FAILED   9/19  mml-secure-en-decrypt-sign-2 (0.629818 sec)
   passed  10/19  mml-secure-find-usable-keys-1 (0.033761 sec)
   passed  11/19  mml-secure-find-usable-keys-2 (0.046812 sec)
   passed  12/19  mml-secure-key-checks (0.064345 sec)
   passed  13/19  mml-secure-select-preferred-keys-1 (0.003727 sec)
   passed  14/19  mml-secure-select-preferred-keys-2 (0.027046 sec)
   passed  15/19  mml-secure-select-preferred-keys-3 (0.027371 sec)
   passed  16/19  mml-secure-select-preferred-keys-4 (0.023920 sec)
   passed  17/19  mml-secure-sign-verify-1 (0.981920 sec)
   passed  18/19  mml-secure-sign-verify-2 (1.427348 sec)
   passed  19/19  mml-secure-sign-verify-3 (0.014458 sec)

Ran 19 tests, 16 results as expected, 1 unexpected, 2 skipped (2020-08-12 18:31:50-0700, 5.894490 sec)
1 expected failures

1 unexpected results:
   FAILED  mml-secure-en-decrypt-sign-2

2 skipped results:
  SKIPPED  mml-secure-en-decrypt-passphrase-cache
  SKIPPED  mml-secure-en-decrypt-passphrase-no-cache-openpgp-todo