Remove clippy deny attributes from test suite

These were superseded by the clippy invocation made in our CI workflow:

    run: cargo clippy --tests -- -Dclippy::all -Dclippy::pedantic

tests/test_source.rs

@@ -1,5 +1,3 @@
-#![deny(clippy::all, clippy::pedantic)]
-
 use std::error::Error as StdError;
 use std::io;
 use thiserror::Error;