tests-extra: valgrind suppression for call_rcu

tests-extra/data/knot.supp

+{
+   liburcu/call_rcu
+   Memcheck:Leak
+   match-leak-kinds: possible,reachable
+   fun:calloc
+   ...
+   fun:call_rcu_memb
+}
+

tests-extra/tools/dnstest/test.py

@@ -167,6 +167,9 @@ class Test(object):
             srv.valgrind = [params.valgrind_bin] + \
                            params.valgrind_flags.split() + \
                            ["--log-file=%s" % srv.valgrind_log]
+            suppressions_file = "%s/%s.supp" % (params.common_data_dir, server)
+            if os.path.isfile(suppressions_file):
+                srv.valgrind.append("--suppressions=%s" % suppressions_file)
 
         self.servers.add(srv)
         return srv