tests: replace write by xwrite

Using write without cheks leads to warn unused result when __wur is
enabled.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

libio/bug-wfflush.c

@@ -3,6 +3,8 @@
 #include <stdio.h>
 #include <wchar.h>
 
+#include <support/xunistd.h>
+
 static void do_prepare (void);
 #define PREPARE(argc, argv) do_prepare ()
 static int do_test (void);
@@ -20,7 +22,7 @@ do_prepare (void)
       printf ("cannot create temporary file: %m\n");
       exit (1);
     }
-  write (fd, "1!", 2);
+  xwrite (fd, "1!", 2);
   close (fd);
 }