Correct test udp_dtls_handshake

A call to

    `UDPSocket::recvfrom(SocketAddress *address, void *data, nsapi_size_t size)`

returns, following the mbed documentation, the number of received bytes on
success, and a negative error code on failure.

So in case of success, the return value depends on both the value of parameter
`size` but also on the amount of data already available. This means, that the
value returned can be lower than or equal to the `size` of the `data` buffer
passed as argument to the call.

Therefore, in the case of `test_udp_dtls_handshake()`, the call to
`sock.recvfrom()` might return up to `sizeof(buffer)` (i.e. 512) bytes, while
the test expects that exactly `udp_dtls_handshake_pattern[step]` bytes are
being received by this call.

This commit applies a *workaround* to this situation by specifying
`udp_dtls_handshake_pattern[step]` as the `size` of the `data` buffer in the
call to `sock.recvfrom()`, which inhibits that a too big amount of bytes gets
received by this call.
This is just a *workaround* as it is still possible that the call to
`sock.recvfrom()` might return less than the number of bytes expected by the
test.

Author: betzw

TESTS/netsocket/udp_dtls_handshake/main.cpp

@@ -108,7 +108,7 @@ void test_udp_dtls_handshake() {
 
         int step = 0;
         while (step < udp_dtls_handshake_count) {
-            err = sock.recvfrom(NULL, buffer, sizeof(buffer));
+            err = sock.recvfrom(NULL, buffer, udp_dtls_handshake_pattern[step]);
             printf("UDP: rx <- %d ", err);
 
             // check length