[Feature request] Support Curio, trio or other async libraries

[logileifs]

Would you be willing to add support for Curio and/or Trio? I would submit a PR myself but I see in https://github.com/darrenburns/ward/blob/master/ward/testing.py#L200 that asyncio.run(coro) is hardcoded and I'm not sure how you would go about to make that configurable. If you could check on the user supplied config before calling asyncio.run(coro) you could do something like:

if self.is_async_test:
    coro = self.fn(**resolved_args)
    if config.asynclib == 'curio':
        import curio
        curio.run(coro)
    elif config.asynclib == 'trio':
        import trio
        trio.run(coro)
    else:
        asyncio.run(coro)
else:
    self.fn(**resolved_args)

[logileifs]

I took a stab at it myself: #357