test: convert scheduler tests to use the new async scheduler

This commit performs the changes necessary to port the scheduler tests
to use the new async scheduler. This involves:
- Creating a Mock RuntimeBackend. For now, to keep changes minimal and
  simple, we just use the LauncherAdapter with the MockLauncher. In the
  future it would be nice to make a mock RuntimeBackend as well though.
- Mark all the tests as being asyncio with async def and use the new
  scheduler interface.
- Update a couple of tests that were weirdly constructed (e.g. in terms
  of targets/ordering) due to constraints of the old scheduler.

With these changes, _all_ scheduler tests are now passing with the new
async scheduler across multiple iterations.

Signed-off-by: Alex Jones <alex.jones@lowrisc.org>

Author: AlexJones0