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These were superseded by the clippy invocation made in our CI workflow: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

@@ -1,4 +1,3 @@
-#![deny(clippy::all, clippy::pedantic)]
 #![allow(clippy::iter_cloned_collect, clippy::option_if_let_else)]
 
 use std::fmt::Display;