>> make[4]: Entering directory '/tmp/nix-build-gnutls-3.3.1.drv-0/gnutls-3.3.1/tests/cert-tests'
>> PASS: pathlen
>> PASS: aki
>> FAIL: pem-decoding
>> SKIP: template-test
>> make[5]: Entering directory '/tmp/nix-build-gnutls-3.3.1.drv-0/gnutls-3.3.1/tests/cert-tests'

> Does test-suite.log provide any useful hints?

Both issues should be fixed in the development version.  Would you like
to see it packaged or wait for the new release?