rust: error: check for config `test` in `Error::name`

Additional to `testlib` also check for `test` in `Error::name`. This is
required by a subsequent patch that (indirectly) uses `Error` in test
cases.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20241004154149.93856-24-dakr@kernel.org


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>