Fix: change tooslow to slow
# What does this PR do?

`test_saved_model_creation_extended` in `TFViT` test is currently `tooslow`. It was copied from TF common test.
#14415 move it from common test to TF core test + changed it to `slow`.

This PR fix this inconsistency.

## Who can review?

@Rocketknight1 
LGTM!