Add timeouts to wait in shutting down in tests

[tagomoris]

Fluent::PluginHelper::Thread#thread_wait_until_stop waits until all threads exit.
Test drivers call this method.

But if there are some reason or bug not to quit threads in plugins, test runner blocks forever.
It (and Ctrl-c also) doesn't show test case names which causes blocking.
So, it's better to raise error for timeout in tests.