2 slow tests fail on macOS

Description of problem:

override-ciphers and test-ciphers.sh tests fail on macOS.

Version of gnutls used:

Failure occurs on GnuTLS 3.6.11.1 - 3.6.13. v3.6.10 passes everything.

Distributor of gnutls (e.g., Ubuntu, Fedora, RHEL)

macOS 10.13.6 (building via Fink package manager)

How reproducible:

Steps to Reproduce:

• Build gnutls with these configure parameters, then make, and run tests with make check.

Actual results:

This is the output from tests/slow/test-suite.log:

FAIL: test-ciphers.sh
=====================

default cipher tests failed
FAIL test-ciphers.sh (exit status: 1)

FAIL: override-ciphers
======================

overridden cipher tests failed
FAIL override-ciphers (exit status: 1)

The individual logs for the 2 failed tests do not show any more details.

Expected results:

All tests should pass. Issue #929 mentions these tests failing, but that's really about some tests are being run even though the --disable-full-test-suite option was used.