Whenever the trio_timeout option is enabled, this plugin will hook into
requests from pytest-timeout to set a timeout. It will then start a
thread in the background that, after the timeout has reached, will
inject a system task in the test loop. This system task will collect
stacktraces for all tasks and raise an exception that will terminate the
test. The timeout thread is reused for other tests as well to not incur
a startup cost for every test.

Since this feature integrates with pytest-timeout, it also honors things
like whether a debugger is attached or not.

Drawbacks:

- Ideally, whether trio does timeouts should not be a global option, but
  would be better suited for the timeout-method in pytest-timeout. This
  would require a change in pytest-timeout to let plugins register other
  timeout methods.
- This method requires a functioning loop.

Fixes python-trio#53

The pytest tester does not run tests in a subinterpreter or subprocess,
so changing trio.run in a testdir test will also change the global
trio.run. Monkeypatching resets trio.run after the test.