test: Mark TEST-21-DFUZZER as slow and skip it by default

The test is not really useful without sanitizers, so let's mark it
as slow and not run it by default in CI.

test/TEST-21-DFUZZER/meson.build

@@ -3,4 +3,5 @@
 test_params +=  {
         'timeout' : 3600,
         'priority' : -50,
+        'slow' : true,
 }