test_exception.rb: ignore SystemStackError

* test/ruby/test_exception.rb (test_machine_stackoverflow_by_define_method):
  ignore propagated SystemStackError, as well as
  test_machine_stackoverflow, which has not bee rescued in the child
  process by unknown reason.

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44753 b2dd03c8-39d4-4d8f-98ff-823fe69b080e