Disable compress-cert-conf test.

In 3.8.10, the test suite doesn't set LD_LIBRARY_PATH, so this runs against the
installed gnutls rather than the one just built, and fails.

--- gnutls-3.8.10/tests/system-override-compress-cert.sh	2025-05-20 09:39:47.000000000 +0100
+++ gnutls-3.8.10/tests/system-override-compress-cert.sh	2025-07-09 11:39:21.537821211 +0100
@@ -19,6 +19,8 @@
 # You should have received a copy of the GNU Lesser General Public License
 # along with this program.  If not, see <https://www.gnu.org/licenses/>
 
+exit 77
+
 TEST=${builddir}/compress-cert-conf
 CONF=config.$$.tmp
 export GNUTLS_SYSTEM_PRIORITY_FILE=${CONF}