test: add test that shows size limitation of jsoncomm

The jsoncomm code is using `socket.SOCK_SEQPACKET` and is written
with the assumption that each send/recv transmit exactly one msg.
Unfortunately there is a size limit for these messages defined
by the kernel (`/proc/sys/net/core/wmem_max`) which can be hit
by large messages, e.g. from a `org.osbuild.files:map()`.

test/mod/test_util_jsoncomm.py

@@ -216,3 +216,14 @@ def test_send_and_recv(self):
         self.assertEqual(ping, pong)
         pong, _, _ = a.recv()
         self.assertEqual(ping, pong)
+
+
+    def test_send_and_recv_lots_of_data(self):
+        a, b = jsoncomm.Socket.new_pair()
+
+        ping = {"data": "much" * 100_000}
+        a.send(ping)
+        pong, _, _ = b.send_and_recv(ping)
+        self.assertEqual(ping, pong)
+        pong, _, _ = a.recv()
+        self.assertEqual(ping, pong)