text: Title: trio.testing.MockClock-like functionality
  Body: some way of jumping forwards in time for asyncio and trio backends, and maybe a pytest-fixture for it too
  
  https://trio.readthedocs.io/en/stable/reference-testing.html#time-and-timeouts
  
  a pytest-asyncio fixture for time travelling: https://github.com/pytest-dev/pytest-asyncio/pull/113
  an asynctest unittest TestCase: https://asynctest.readthedocs.io/en/latest/_modules/asynctest/case.html#ClockedTestCase.advance