test/ruby: fix leaked threads

* test/thread/test_backtrace.rb: join work threads not to leak
  threads.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46132 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test_http.rb: fix leaked threads

* test/net/http/test_http.rb (test_timeout_during_HTTP_session):
  join work threads not to leak threads.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46133 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test_imap.rb: fix leaked threads

* test/net/imap/test_imap.rb (imaps_test): join work threads not
  to leak threads.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46134 b2dd03c8-39d4-4d8f-98ff-823fe69b080e