test_string.rb: enable huge test only on possible platforms

* test/ruby/test_string.rb (test_LSHIFT_neary_long_max): enable
  only on platforms where string size range is smaller than memory
  space.  this test does not make sense but just wastes memory and
  time on other platforms, as it is hardly possible that a string
  size becomes neary LONG_MAX if long size equals pointer size.
  [ruby-core:65410] [Bug #10325]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47817 b2dd03c8-39d4-4d8f-98ff-823fe69b080e