wait for the process

[graingert]

fixes #571

when using the async_playwright() context manager early in an asyncio.run program, the proc can be garbage collected after the loop is closed. When proc is garbage collected the underlying transport is then garbage collected, which tries to kill the running process but cannot without a running loop

[pavelfeldman]

Thanks for the PR! I can imagine writing the test for this could be problematic due to GC, but scoping process to the event loop lifetime makes sense.