Fix clippy errors on tests (#2160)

## Issue Addressed

There are some clippy error on tests.


## Proposed Changes

Enable clippy check on tests and fix the errors. 💪

beacon_node/eth1/src/deposit_cache.rs

@@ -397,7 +397,7 @@ pub mod tests {
             .expect("should get the four from the full tree");
         assert_eq!(
             deposits.len(),
-            4 as usize,
+            4_usize,
             "should get 4 deposits from full tree"
         );
         assert_eq!(
@@ -418,7 +418,7 @@ pub mod tests {
             .expect("should get the half tree");
         assert_eq!(
             deposits.len(),
-            4 as usize,
+            4_usize,
             "should get 4 deposits from half tree"
         );
         assert_eq!(

beacon_node/eth1/src/service.rs

@@ -1179,7 +1179,6 @@ async fn download_eth1_block(
 #[cfg(test)]
 mod tests {
     use super::*;
-    use toml;
     use types::MainnetEthSpec;
 
     #[test]