test: the take-back byte is read back off the wire

ipc_response_fragmented_and_truncated already packed del = 2 and sent
the frame a byte at a time; it checked the commit and the preedit and
never looked at the field the v2 wire was widened for.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/ipc_test.c

@@ -302,6 +302,7 @@ ipc_response_fragmented_and_truncated(struct ct *t)
 		}
 	if(i == n && CT_EQ_INT(t, 0, ipcreadresp(fd[1], 1, commit, preedit,
 	   &resp))){
+		CT_EQ_INT(t, 2, resp.del);
 		CT_EQ_STR(t, "abc", commit);
 		CT_EQ_STR(t, "xy", preedit);
 	}