On Debian Testing with libseccomp 2.5.1-1, 'make test/emacs-tests' fails
with:

  Test emacs-tests/seccomp/allows-stdout backtrace:
    signal(ert-test-failed (((should (eql status 0)) :form (eql "Bad sys
    ert-fail(((should (eql status 0)) :form (eql "Bad system call" 0) :v
    [...]
    normal-top-level()
  Test emacs-tests/seccomp/allows-stdout condition:
      Info: Process output:
      (ert-test-failed
       ((should
         (eql status 0))
        :form
        (eql "Bad system call" 0)
        :value nil))
     FAILED  2/6  emacs-tests/seccomp/allows-stdout (0.032159 sec)

Investigating a bit further, from the source tree root:

0. cd src
1. gdb ./emacs
2. set logging on
3. b pdumper.c:5263
4. r -Q -batch -seccomp ../test/src/emacs-resources/seccomp-filter.bpf
5. s RET RET RET
6. q

Here's the log: