Skip circuit config / regeneration with test_only feature

[Nashtare]

The test_only feature allows us to skip actual proof generation and quickly test:

• the decoder behavior
• the witness generation

With an emphasis on the latter for debugging, by toggling RUST_LOG to the lowest level.
As such, we don't need any preprocessed circuits the prover may have generated. Since the circuit consistency work has been merged, this has us wiping out the whole circuits/ folder and rewrite new ones (usually set with minimal ranges for speed).

We could actually skip this step altogether, and take the existing circuits as they are, as they won't be at use anyway.
This, combined with a default use of --load-stragegy on-demand upon activation of the test_only flag would remove all needless overhead from preprocessing when testing / debugging, maintain low memory usage, and allow to persist the existing set of circuits (if any) on disk for later use.