tempfile.rb: fix r47655

* lib/tempfile.rb (Tempfile#initialize, Tempfile.create): get rid of
  shadowing local variables.

* lib/tmpdir.rb (Dir::Tmpname#make_tmpname): simlify argument
  splitting.

* test/test_tempfile.rb: need thread library for ConditionVariable.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47656 b2dd03c8-39d4-4d8f-98ff-823fe69b080e