Do not trigger trailing_empty_array in tests (#13844)

Close #13837

changelog: [`trailing_empty_array`]: do not trigger on tests

tests/ui/trailing_empty_array.rs

@@ -193,3 +193,17 @@ type C = ConstParamNoDefault<0>;
 type D = ConstParamNonZeroDefault<0>;
 
 fn main() {}
+
+#[cfg(test)]
+mod tests {
+    pub struct Friend {
+        age: u8,
+    }
+
+    #[test]
+    fn oldest_empty_is_none() {
+        struct Michael {
+            friends: [Friend; 0],
+        }
+    }
+}