trio.testing.MockClock-like functionality

[graingert]

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: pytest-dev/pytest-asyncio#113
an asynctest unittest TestCase: https://asynctest.readthedocs.io/en/latest/_modules/asynctest/case.html#ClockedTestCase.advance