add a test

Author: hauntsaninja

test-data/unit/check-errorcodes.test

@@ -807,7 +807,7 @@ from typing_extensions import TypedDict
 
 Foo = TypedDict("Bar", {})  # E: First argument "Bar" to TypedDict() does not match variable name "Foo"  [name-match]
 [builtins fixtures/dict.pyi]
-[case testImplicitBool]
+[case testTruthyBool]
 # flags: --enable-error-code truthy-bool
 from typing import List, Union
 
@@ -837,15 +837,21 @@ if s:
 good_union: Union[str, int] = 5
 if good_union:
     pass
+if not good_union:
+    pass
 
 bad_union: Union[Foo, object] = Foo()
 if bad_union:  # E: "__main__.bad_union" has type "Union[__main__.Foo, builtins.object]" of which no members implement __bool__ or __len__ so it could always be true in boolean context  [truthy-bool]
     pass
+if not bad_union:
+    pass
 
 def f():
     pass
 if f:  # E: Function "def () -> Any" could always be true in boolean context  [truthy-bool]
     pass
+if not f:
+    pass
 conditional_result = 'foo' if f else 'bar'  # E: Function "def () -> Any" could always be true in boolean context  [truthy-bool]
 
 lst: List[int] = []