Fixes findings from Greentea netsocket UDP test cases

TESTS/netsocket/udp/udpsocket_echotest.cpp

@@ -67,6 +67,7 @@ void UDPSOCKET_ECHOTEST()
         fill_tx_buffer_ascii(tx_buffer, BUFF_SIZE);
 
         for (int retry_cnt = 0; retry_cnt <= 2; retry_cnt++) {
+            memset(rx_buffer, 0, BUFF_SIZE);
             sent = sock.sendto(udp_addr, tx_buffer, pkt_s);
             if (sent != pkt_s) {
                 printf("[Round#%02d - Sender] error, returned %d\n", s_idx, sent);

TESTS/netsocket/udp/udpsocket_echotest_burst.cpp

@@ -23,20 +23,23 @@
 #include "utest.h"
 #include "udp_tests.h"
 
-#define SIGNAL_SIGIO 0x1
-#define SIGIO_TIMEOUT 5000 //[ms]
-#define RECV_TIMEOUT 1 //[s]
+using namespace utest::v1;
 
 namespace
 {
-    typedef struct pkg {
-        int len;
-        char *payload;
-    } pkg_t;
+    static const int SIGNAL_SIGIO = 0x1;
+    static const int SIGIO_TIMEOUT = 5000; //[ms]
+    static const int RECV_TIMEOUT = 1; //[s]
+
     static const int BURST_CNT = 100;
     static const int BURST_PKTS = 5;
     static const int PKG_SIZES[BURST_PKTS] = {100, 200, 300, 120, 500};
     static const int RECV_TOTAL = 1220;
+
+    typedef struct pkg {
+        int len;
+        char *payload;
+    } pkg_t;
     pkg_t tx_buffers[BURST_PKTS];
     char rx_buffer[500] = {0};
 }

TESTS/netsocket/udp/udpsocket_open_close_repeat.cpp

@@ -23,6 +23,8 @@
 #include "unity/unity.h"
 #include "utest.h"
 
+using namespace utest::v1;
+
 void UDPSOCKET_OPEN_CLOSE_REPEAT()
 {
     UDPSocket *sock = new UDPSocket;

TESTS/netsocket/udp/udpsocket_recv_timeout.cpp

@@ -63,7 +63,7 @@ void UDPSOCKET_RECV_TIMEOUT()
         if (recvd == NSAPI_ERROR_WOULD_BLOCK) {
             osSignalWait(SIGNAL_SIGIO, SIGIO_TIMEOUT);
             printf("MBED: recvfrom() took: %dms\n", timer.read_ms());
-            TEST_ASSERT_INT_WITHIN(50, (100+200)/2, timer.read_ms());
+            TEST_ASSERT_INT_WITHIN(50, 150, timer.read_ms());
             continue;
         } else if (recvd < 0) {
             printf("[bt#%02d] network error %d\n", i, recvd);

TESTS/netsocket/udp/udpsocket_sendto_timeout.cpp

@@ -23,6 +23,8 @@
 #include "utest.h"
 #include "udp_tests.h"
 
+using namespace utest::v1;
+
 void UDPSOCKET_SENDTO_TIMEOUT()
 {
     char tx_buffer[100];