net: sockets: udp: test FIONREAD with ipv4 and ipv6 sockets

The FIONREAD ioctl (or rather ZFD_IOCTL_FIONREAD) returns the
number of bytes available on the socket that can be read
immediately.

Test that the ioctl behaves according to the expectation.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>

tests/net/socket/udp/src/main.c

@@ -58,6 +58,7 @@ static void comm_sendto_recvfrom(int client_sock,
 				 struct sockaddr *server_addr,
 				 socklen_t server_addrlen)
 {
+	int avail;
 	ssize_t sent = 0;
 	ssize_t recved = 0;
 	struct sockaddr addr;
@@ -76,6 +77,12 @@ static void comm_sendto_recvfrom(int client_sock,
 		      0, server_addr, server_addrlen);
 	zassert_equal(sent, strlen(TEST_STR_SMALL), "sendto failed");
 
+	k_msleep(100);
+
+	avail = 42;
+	zassert_ok(ioctl(server_sock, ZFD_IOCTL_FIONREAD, &avail));
+	zassert_equal(avail, strlen(TEST_STR_SMALL));
+
 	/* Test recvfrom(MSG_PEEK) */
 	addrlen = sizeof(addr);
 	clear_buf(rx_buf);