Restricting IPV4 echo tests to chars 0-9.

Having the echo tests generate any random character
caused issues serializing the test output into an
xml file. This change limits the characters generated
by the device to '0' - '9'.

Author: bridadan

features/net/FEATURE_IPV4/TESTS/mbedmicro-net/tcp_client_echo/main.cpp

@@ -21,7 +21,7 @@ namespace {
 
 void prep_buffer(char *tx_buffer, size_t tx_size) {
     for (size_t i=0; i<tx_size; ++i) {
-        tx_buffer[i] = (rand() % ASCII_MAX) + ' ' + 1;
+        tx_buffer[i] = (rand() % 10) + '0';
     }
 }
 

features/net/FEATURE_IPV4/TESTS/mbedmicro-net/udp_echo_client/main.cpp

@@ -20,7 +20,7 @@ namespace {
 
 void prep_buffer(char *tx_buffer, size_t tx_size) {
     for (size_t i=0; i<tx_size; ++i) {
-        tx_buffer[i] = (rand() % ASCII_MAX) + ' ' + 1;
+        tx_buffer[i] = (rand() % 10) + '0';
     }
 }