Fix the test permission of "test_rm_rf"

The test was added for [Bug #6756]. The ticket insisted
`FileUtils.rm_rf` should delete an empty directory even if its
permission is 000. However, the test tried to delete a directory with
permission 700.

Author: mame

test/fileutils/test_fileutils.rb

@@ -1790,7 +1790,7 @@ def test_rm_rf
     return if /mswin|mingw/ =~ RUBY_PLATFORM
 
     mkdir 'tmpdatadir'
-    chmod 0o700, 'tmpdatadir'
+    chmod 0o000, 'tmpdatadir'
     rm_rf 'tmpdatadir'
 
     assert_file_not_exist 'tmpdatadir'