test_beginendblock.rb: assert_normal_exit

* test/ruby/test_beginendblock.rb (test_callcc_at_exit): use
  assert_normal_exit for better message.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43690 b2dd03c8-39d4-4d8f-98ff-823fe69b080e