Ensure we build if a downstream crate sets `--cfg=fuzzing`

Ensure we build if a downstream crate sets `--cfg=fuzzing`

Downstream crates building fur fuzzing will usually set
`--cfg=fuzzing` as a side-effect of the Rust fuzzing tooling. Thus,
we should ensure we build without failure in such cases.

We do this here by simply relying on the `_test_utils` feature,
rather than conditionally-compiling in modules based on the
`fuzzing` flag.