test_gc.rb: termsig

* test/ruby/test_gc.rb (test_interrupt_in_finalizer): also check
  termsig to ensure killed by SIGINT.  and try SIGSEGV to dump
  something.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48974 b2dd03c8-39d4-4d8f-98ff-823fe69b080e