test_rubyoptions.rb: remove core

* test/ruby/test_rubyoptions.rb (test_segv_loaded_features): run
  SEGV test in a temproray directory so that core files may not be
  left, not to confuse chkbuild.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44408 b2dd03c8-39d4-4d8f-98ff-823fe69b080e