test_file_exhaustive.rb: fix expected value

* test/ruby/test_file_exhaustive.rb (test_expand_path_memsize):
  correct expected value, count terminator byte.  [Bug #9934]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46437 b2dd03c8-39d4-4d8f-98ff-823fe69b080e