Test suite for backends? #30

Do we want to add a typing test suite that backends can vendor into their own code to verify that they're compliant with our protocols? Wouldn't need to be too long. Just something like:

# Typing test for Array API version 1234.
import jax.numpy as xp
import array_api_typing as xpt

def f(xp: xpt.ArrayNamespace) -> None:
  pass

f(xp)  # Test that xp is compliant.
# etc.

They can copy that snippet into their tests and verify it using the static type checker. They will need to also ensure that they have array-api-typing==1234 as a test requirement.