test_thread_timer_and_interrupt, test_thread_join_in_trap,
test_thread_join_current, test_thread_join_main_thread,
test_main_thread_status_at_exit, test_thread_status_in_trap,
test_thread_status_raise_after_kill, test_mutex_owned,
test_mutex_owned2): move these tests from TestThreadGroup class
  to TestThread becuase they are not thread group tests.

* test/ruby/test_thread.rb (test_thread_status_raise_after_kill):
  add t.join.

* test/ruby/test_threadgroup.rb: new file. moved ThreadGroup test
  form test_thread.rb.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38388 b2dd03c8-39d4-4d8f-98ff-823fe69b080e