Add a test_sponge public function

[mmagician]

For documentation and testing purposes, it would be great to obtain a sponge without much code overhead.
The simplest thing to do is to move poseidon_sponge_for_test methods from ark-poly-commit, which are pub(crate) functions for testing there, and to re-expose them under test_sponge in the root module here.

Alternatively this could go into ark-std, similar to test_rng: https://github.com/arkworks-rs/std/blob/4cee4bc18ff6b0159c00d48622181db145938601/src/rand_helper.rs#L37