Fix easy clippy lints from latest stable (#16053)

1.88.0 was released today, clippy now lints (machine-applicable)
against:
- format strings with empty braces that could be inlined
  - easy win
- `manual_abs_diff`
- returning of a stored result of the last expression.
  - this can be somewhat contentious but touched only a few places

crates/nu-plugin-core/src/communication_mode/local_socket/tests.rs

@@ -5,7 +5,7 @@ fn local_socket_path_contains_pid() {
     let name = make_local_socket_name("test-string")
         .to_string_lossy()
         .into_owned();
-    println!("{}", name);
+    println!("{name}");
     assert!(name.to_string().contains(&std::process::id().to_string()));
 }
 
@@ -14,6 +14,6 @@ fn local_socket_path_contains_provided_name() {
     let name = make_local_socket_name("test-string")
         .to_string_lossy()
         .into_owned();
-    println!("{}", name);
+    println!("{name}");
     assert!(name.to_string().contains("test-string"));
 }

crates/nu-plugin-core/src/interface/stream/tests.rs

@@ -201,7 +201,7 @@ fn reader_drop() {
 
     impl WriteStreamMessage for Check {
         fn write_stream_message(&mut self, msg: StreamMessage) -> Result<(), ShellError> {
-            assert!(matches!(msg, StreamMessage::Drop(1)), "got {:?}", msg);
+            assert!(matches!(msg, StreamMessage::Drop(1)), "got {msg:?}");
             self.0.store(true, Relaxed);
             Ok(())
         }

crates/nu-plugin-core/src/serializers/json.rs

@@ -122,8 +122,7 @@ mod tests {
         let string = std::str::from_utf8(&out).expect("utf-8 error");
         assert!(
             string.ends_with('\n'),
-            "doesn't end with newline: {:?}",
-            string
+            "doesn't end with newline: {string:?}"
         );
     }