Fix ignored clippy lints (#12160)

# Description
Fixes some ignored clippy lints.

# User-Facing Changes
Changes some signatures and return types to `&dyn Command` instead of
`&Box<dyn Command`, but I believe this is only an internal change.

crates/nu-command/tests/commands/table.rs

@@ -2346,7 +2346,6 @@ fn join_lines(lines: impl IntoIterator<Item = impl AsRef<str>>) -> String {
 }
 
 // util function to easier copy && paste
-#[allow(dead_code)]
 fn _print_lines(s: &str, w: usize) {
     eprintln!("{:#?}", _split_str_by_width(s, w));
 }

crates/nu-command/tests/commands/where_.rs

@@ -1,5 +1,5 @@
 use nu_test_support::nu;
-#[allow(unused_imports)]
+#[cfg(feature = "sqlite")]
 use nu_test_support::pipeline;
 
 #[test]

crates/nu-command/tests/format_conversions/csv.rs

@@ -183,7 +183,6 @@ fn from_csv_text_with_tab_separator_to_table() {
 }
 
 #[test]
-#[allow(clippy::needless_raw_string_hashes)]
 fn from_csv_text_with_comments_to_table() {
     Playground::setup("filter_from_csv_test_5", |dirs, sandbox| {
         sandbox.with_files(vec![FileWithContentToBeTrimmed(
@@ -377,7 +376,7 @@ fn from_csv_text_with_wrong_type_separator() {
 fn table_with_record_error() {
     let actual = nu!(pipeline(
         r#"
-            [[a b]; [1 2] [3 {a: 1 b: 2}]] 
+            [[a b]; [1 2] [3 {a: 1 b: 2}]]
             | to csv
         "#
     ));

crates/nu-command/tests/format_conversions/tsv.rs

@@ -106,7 +106,6 @@ fn from_tsv_text_to_table() {
 }
 
 #[test]
-#[allow(clippy::needless_raw_string_hashes)]
 fn from_tsv_text_with_comments_to_table() {
     Playground::setup("filter_from_tsv_test_2", |dirs, sandbox| {
         sandbox.with_files(vec![FileWithContentToBeTrimmed(