text: Title: Improve tests for `piccolo asgi new`
  Body: We have a basic test for `piccolo asgi new`:
  
  https://github.com/piccolo-orm/piccolo/blob/master/tests/apps/asgi/commands/test_new.py
  
  It can definitely be improved though. As a minimum, we should read the generated file contents, and use `ast.parse(file_contents)` to make sure the file is valid Python code. We use a similar approach here:
  
  https://github.com/piccolo-orm/piccolo/blob/e0f04a40e868e9fa3c4f6bb9ebb1128f74180b07/tests/apps/schema/commands/test_generate.py#L91
  
  Even better, we would try and run the app to make sure it works, but this might be too tricky.
  