add test for &str

tests/ui/str_to_string.rs

@@ -1,5 +1,7 @@
 #![warn(clippy::str_to_string)]
 
 fn main() {
-    let _ = "hello world".to_string();
+    let hello = "hello world".to_string();
+    let msg = &hello[..];
+    msg.to_string();
 }

tests/ui/str_to_string.stderr

@@ -1,11 +1,19 @@
 error: `to_string()` does a multitude of things to just clone a `&str`
-  --> $DIR/str_to_string.rs:4:13
+  --> $DIR/str_to_string.rs:4:17
    |
-LL |     let _ = "hello world".to_string();
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     let hello = "hello world".to_string();
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D clippy::str-to-string` implied by `-D warnings`
    = help: consider using `.to_owned()`
 
-error: aborting due to previous error
+error: `to_string()` does a multitude of things to just clone a `&str`
+  --> $DIR/str_to_string.rs:6:5
+   |
+LL |     msg.to_string();
+   |     ^^^^^^^^^^^^^^^
+   |
+   = help: consider using `.to_owned()`
+
+error: aborting due to 2 previous errors