Fix unused import clippy lint

There is an unused import when run with --no-default-features.

Feature gate the import correctly.

Author: jamillambert

units/src/amount/tests.rs

@@ -11,7 +11,9 @@ use core::num::{NonZeroI64, NonZeroU64};
 use std::panic;
 
 use super::*;
-use crate::result::{MathOp, NumOpResult};
+use crate::result::NumOpResult;
+#[cfg(feature = "alloc")]
+use crate::result::MathOp;
 #[cfg(feature = "alloc")]
 use crate::{FeeRate, Weight};