Modify serial nc tests init part

During initialization phase of the 2 NC serial tests,
there is a character being sent from host to target,
based on MBED_HOSTTEST_START. The problem is that
the target firmware then creates a "new" serial on the
same serial interface. the new or should, or at least may,
induce a reset of the IP, so that the character sent by host
would be lost. So we're moving the new earlier.

the real testing part happens later, to check the next
character is not received or sent.

libraries/tests/mbed/serial_nc_rx/main.cpp

@@ -2,13 +2,13 @@
 #include "test_env.h"
 
 int main() {
+    Serial *pc = new Serial(USBTX, USBRX);
+
     MBED_HOSTTEST_TIMEOUT(20);
     MBED_HOSTTEST_SELECT(serial_nc_rx_auto);
     MBED_HOSTTEST_DESCRIPTION(Serial NC RX);
     MBED_HOSTTEST_START("MBED_37");
 
-    Serial *pc = new Serial(NC, USBRX);
-
     char c = pc->getc();
 
     delete pc;

libraries/tests/mbed/serial_nc_tx/main.cpp

@@ -2,13 +2,14 @@
 #include "test_env.h"
 
 int main() {
+    Serial *pc = new Serial(USBTX, USBRX);
+
     MBED_HOSTTEST_TIMEOUT(20);
     MBED_HOSTTEST_SELECT(serial_nc_tx_auto);
     MBED_HOSTTEST_DESCRIPTION(Serial NC TX);
     MBED_HOSTTEST_START("MBED_38");
 
     // Wait until we receive start signal from host test
-    Serial *pc = new Serial(USBTX, USBRX);
     char c = pc->getc();
     delete pc;