zdtm: don't use dprintf, because it's not thread safe

pthread00 hangs if dprintf is used, and works well if fprintf is used.

One more mention of this bug is here:
http://sourceware.org/bugzilla/show_bug.cgi?id=12847Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm/lib/msg.c

@@ -56,5 +56,5 @@ void test_msg(const char *format, ...)
 	len = vsnprintf(buf + off, sizeof(buf) - off, format, arg);
 	va_end(arg);
 
-	dprintf(STDERR_FILENO, buf);
+	fprintf(stderr, buf);
 }