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_bytes.c

@@ -0,0 +1,42 @@
+/* Compare bytes 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 <support/file_contents.h>
+
+/* Check that an already-open file has exactly the given bytes,
+   starting at the current offset.  */
+
+int
+support_compare_file_bytes (FILE *fp, const char *contents, size_t length)
+{
+  int c;
+  while (length > 0)
+    {
+      c = getc (fp);
+      if (c == EOF || (unsigned char) c != (unsigned char) contents[0])
+	return 1;
+      contents++;
+      length--;
+    }
+  c = getc (fp);
+  if (c != EOF || ferror (fp))
+    return 1;
+  return 0;
+}