message: Make with_c_api a no-op if the C API is already enabled.
  This will help transition to turning the C API on by default by
  preventing new tests from breaking.
diff: @@ -419,6 +419,11 @@ def with_c_api(cls):
  Returns:
  cls with new test methods added
  """
  + # If the C API is already enabled, don't do anything. Some tests break if the
  + # same test is run twice, so this allows us to turn on the C API by default
  + # without breaking these tests.
  + if ops._USE_C_API: return cls
  +
  for name, value in cls.__dict__.copy().items():
  if callable(value) and name.startswith("test"):
  setattr(cls, name + "WithCApi", enable_c_api(value))
  