test_exception.rb: enclose hash defininition

* test/ruby/test_exception.rb: (test_machine_stackoverflow): enclose
  hash defininition because SystemStackError raises at line 3 on
  FreeBSD, for unknown reason.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43735 b2dd03c8-39d4-4d8f-98ff-823fe69b080e