add test

Author: patrickzbhe

extra_tests/snippets/syntax_del.py

@@ -29,3 +29,11 @@ def __del__(self):
 def test_del_panic():
     mytest = MyTest()
     del mytest
+
+# see https://github.com/RustPython/RustPython/issues/4910
+
+def f():
+    del l
+
+l = 'a'
+assert_raises(UnboundLocalError, f)