tests: skip Deckard integration tests if sendmmsg is enabled

All Deckard tests would fail anyway so we now print a warning and skip
Deckard tests.

tests/meson.build

@@ -37,7 +37,11 @@ if build_extra_tests
   py3_deps = []
 
   subdir('pytests')
-  subdir('integration')
+  if not sendmmsg
+    subdir('integration')
+  else
+    warning('Integration tests do not work with sendmmsg, disable sendmmsg first!')
+  endif
 
   foreach py3_dep : py3_deps
     py3_import = run_command(python3, '-c', 'import @0@'.format(py3_dep[0]))