test_thread.rb: move thread switch test

* test/ruby/test_thread.rb (test_switch_while_busy_loop): move
  from test/test_timeout.rb.  [Bug #1402]

* test/test_timeout.rb (test_timeout): no longer related to
  [Bug #1402].  [Bug #8523]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46196 b2dd03c8-39d4-4d8f-98ff-823fe69b080e