Add avtest support for arbitrary commands

This is useful e.g. invalid signal identifier AVDTP tests.
diff --git a/test/avtest.c b/test/avtest.c
index fb9d94a..9e1ef99 100644
--- a/test/avtest.c
+++ b/test/avtest.c
@@ -390,6 +390,13 @@
 		buf[2] = 1 << 2; /* ACP SEID */
 		len = write(sk, buf, 3);
 		break;
+
+	default:
+		hdr->message_type = AVDTP_MSG_TYPE_COMMAND;
+		hdr->packet_type = AVDTP_PKT_TYPE_SINGLE;
+		hdr->signal_id = cmd;
+		len = write(sk, buf, 2);
+		break;
 	}
 
 	len = read(sk, buf, sizeof(buf));