test-qcdm-serial-port: fix warnings with -Wsign-compare

test-qcdm-serial-port.c: In function ‘print_buf’:
  test-qcdm-serial-port.c:79:19: error: comparison of integer expressions of different signedness: ‘int’ and ‘gsize’ {aka ‘long unsigned int’} [-Werror=sign-compare]
     79 |     for (i = 0; i < len; i++) {
        |                   ^
  test-qcdm-serial-port.c: In function ‘server_wait_request’:
  test-qcdm-serial-port.c:163:20: error: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Werror=sign-compare]
    163 |     } while (total < sizeof (readbuf));
        |                    ^

src/tests/test-qcdm-serial-port.c

@@ -72,7 +72,7 @@ wait_for_child (TestData *d, guint32 timeout)
 static void
 print_buf (const char *detail, const char *buf, gsize len)
 {
-    int i = 0;
+    guint i = 0;
     gboolean newline = FALSE;
 
     g_print ("%s (%zu)  ", detail, len);
@@ -116,7 +116,7 @@ server_wait_request (int fd, char *buf, gsize len)
     struct timeval timeout = { 1, 0 };
     char readbuf[1024];
     ssize_t bytes_read;
-    int total = 0, retries = 0;
+    guint total = 0, retries = 0;
     gsize decap_len = 0;
 
     FD_ZERO (&in);