tests: also skip qemuagenttest with old jansson

qemuagenttest also depends on JSON object key ordering:
Invalid value of argument 'vcpus' of command 'guest-set-vcpus':
expected '[{"logical-id":1,"online":false}]' got '[{"online":false,"logical-id":1}]'

Skip it as well.
Signed-off-by: NJán Tomko <jtomko@redhat.com>

tests/qemuagenttest.c

@@ -907,8 +907,8 @@ mymain(void)
 {
     int ret = 0;
 
-#if !WITH_JANSSON
-    fputs("libvirt not compiled with JSON support, skipping this test\n", stderr);
+#if !WITH_STABLE_ORDERING_JANSSON
+    fputs("libvirt not compiled with recent enough Jansson, skipping this test\n", stderr);
     return EXIT_AM_SKIP;
 #endif