Add support/ code for checking file contents

For use in freopen tests, add various support/ helper interfaces for
use in checking file contents.

Tested for x86_64.

support/support_compare_file_string.c

@@ -0,0 +1,28 @@
+/* Compare string from an open file.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+#include <string.h>
+
+#include <support/file_contents.h>
+
+int
+support_compare_file_string (FILE *fp, const char *contents)
+{
+  return support_compare_file_bytes (fp, contents, strlen (contents));
+}