Add a test for BTreeMap with zero-sized value type

Author: korrat

tests/ui/zero_sized_map_values.rs

@@ -1,5 +1,5 @@
 #![warn(clippy::zero_sized_map_values)]
-use std::collections::HashMap;
+use std::collections::{BTreeMap, HashMap};
 
 const CONST_OK: Option<HashMap<String, usize>> = None;
 const CONST_NOT_OK: Option<HashMap<String, ()>> = None;
@@ -66,4 +66,5 @@ fn main() {
     let _: HashMap<String, usize> = HashMap::new();
 
     let _: HashMap<_, _> = std::iter::empty::<(String, ())>().collect();
+    let _: BTreeMap<_, _> = std::iter::empty::<(String, ())>().collect();
 }

tests/ui/zero_sized_map_values.stderr

@@ -103,5 +103,13 @@ LL |     let _: HashMap<_, _> = std::iter::empty::<(String, ())>().collect();
    |
    = help: consider using a set instead
 
-error: aborting due to 13 previous errors
+error: map with zero-sized value type
+  --> $DIR/zero_sized_map_values.rs:69:12
+   |
+LL |     let _: BTreeMap<_, _> = std::iter::empty::<(String, ())>().collect();
+   |            ^^^^^^^^^^^^^^
+   |
+   = help: consider using a set instead
+
+error: aborting due to 14 previous errors