text: Title: Expose dtypes from fused_numerics.pxd for test suite
  Body: ### Description:
  
  We have a nice collection of fused numeric types in [`fused_numerics.pxd`](https://github.com/scikit-image/scikit-image/blob/main/skimage/_shared/fused_numerics.pxd). 
  
  - [ ] It would be nice to have corresponding lists of these fused types available at runtime from Python. That way we can easily slap the appropriate `@pytest.mark.parametrize("dtype", _dtypes_uint)` on a test instead of listing each dtype by hand again and again. Could go into `_shared.testing` in my opinion.
  - [ ] Check that we used these consistently instead of creating a new local fused type each time. Looking at you [`_extrema_cy.pyx`](https://github.com/scikit-image/scikit-image/blob/e964c61663b0ab515955fd17b997de24f881b66c/skimage/morphology/_extrema_cy.pyx#L18-L28).