Meson: validate-metainfo: add --no-net option

Sometimes I work without internet connection, and I still want ninja
test to succeed.

data/meson.build

@@ -20,7 +20,7 @@ if appstreamcli.found()
   test(
     'validate-metainfo',
     appstreamcli,
-    args: ['validate', metainfo_file]
+    args: ['validate', '--no-net', metainfo_file]
   )
 endif