tests: avoid spurious failure when VERSION_CONTROL envvar is set

* tests/backup01.at: Unset VERSION_CONTROL.  Otherwise,
when set to e.g., 'always', it would cause this test to fail.

tests/backup01.at

@@ -1,7 +1,7 @@
 # Process this file with autom4te to create testsuite. -*- Autotest -*-
 
 # Test suite for GNU tar.
-# Copyright (C) 2009 Free Software Foundation, Inc.
+# Copyright (C) 2009-2010 Free Software Foundation, Inc.
 
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -34,6 +34,7 @@ AT_SETUP([extracting existing dir with --backup])
 AT_KEYWORDS([extract backup backup01])
 
 AT_TAR_CHECK([
+unset VERSION_CONTROL
 mkdir dir1 dir2
 echo bla > dir1/file1
 tar cf test.tar dir1 dir2