test/ruby/test_env.rb: test memory leaks only on Windows

* test/ruby/test_env.rb (test_memory_leak_{aset,select,shift}):
  test for Windows platform specific code.
  (test_memory_crash_select): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46822 b2dd03c8-39d4-4d8f-98ff-823fe69b080e