text: Title: Support inline flags in test cases
  Body: Currently, our test cases are executed with different flags based on some combination of their filename and what directory they are in. I would prefer a system where we define the flags in a directive inline, e.g. like this:
  
  ```
  # flags: --target-version=py38 --preview
  1+1
  
  # output
  1 + 1
  ```
  
  