Fix GnuTLS test suite with GnuTLS versions 3.4.x

Fix GnuTLS test suite with GnuTLS versions 3.4.x

* src/gnutls.c (gnutls_cipher_get_tag_size): Make it return
zero only for versions of GnuTLS < 3.2.2, where
gnutls_cipher_get_tag_size was introduced.  This fixes the
GnuTLS test suite, which assumes that any cipher whose tag
size is non-zero is AEAD-capable, and doesn't test such ciphers
if AEAD is not available, i.e. for GnuTLS < 3.5.1.  (Bug#32446)