Fix test for tile method in tests/test_assets_reader.py (very slow)

I skip the test for the tile method of the AssetsReader because it was very slow (but passing): https://github.com/developmentseed/titiler-stacapi-mspc/blob/main/tests/test_assets_reader.py#L48-L56

This task is to look into a way to test the method that is faster, changes to the assertion are ok as long as it's tested that at least the inner tile method on the Reader|Xarray reader is called