Add additional test cases for F823 (#6036)

Making some behavior explicit / codified. See:
https://github.com/astral-sh/ruff/issues/6029.

crates/ruff/resources/test/fixtures/pyflakes/F823.py

@@ -39,3 +39,27 @@ class Class:
     def f(self):
         print(my_var)
         my_var = 1
+
+
+import sys
+
+
+def main():
+    print(sys.argv)
+
+    try:
+        3 / 0
+    except ZeroDivisionError:
+        import sys
+
+        sys.exit(1)
+
+
+import sys
+
+
+def main():
+    print(sys.argv)
+
+    for sys in range(5):
+        pass

crates/ruff/src/rules/pyflakes/rules/undefined_local.rs

@@ -33,7 +33,7 @@ use crate::checkers::ast::Checker;
 /// ```
 #[violation]
 pub struct UndefinedLocal {
-    pub name: String,
+    name: String,
 }
 
 impl Violation for UndefinedLocal {

crates/ruff/src/rules/pyflakes/snapshots/ruff__rules__pyflakes__tests__F823_F823.py.snap

@@ -25,4 +25,22 @@ F823.py:40:15: F823 Local variable `my_var` referenced before assignment
 41 |         my_var = 1
    |
 
+F823.py:48:11: F823 Local variable `sys` referenced before assignment
+   |
+47 | def main():
+48 |     print(sys.argv)
+   |           ^^^ F823
+49 | 
+50 |     try:
+   |
+
+F823.py:62:11: F823 Local variable `sys` referenced before assignment
+   |
+61 | def main():
+62 |     print(sys.argv)
+   |           ^^^ F823
+63 | 
+64 |     for sys in range(5):
+   |
+