Add a test for sentinel assigment

Signed-off-by: Edgar Ramírez Mondragón <edgarrm358@gmail.com>

Author: edgarrmondragon

test-data/unit/check-sentinels.test

@@ -216,3 +216,16 @@ def func(d: dict[str, str]) -> None:
     var = d.get("key", Unknown)
     x: int = var  # E: Incompatible types in assignment (expression has type "str | Unknown", variable has type "int")
 [builtins fixtures/dict-full.pyi]
+
+[case testAssignSentinel]
+from typing_extensions import sentinel
+
+X = sentinel("X")
+
+def inspect_sentinel(arg: sentinel) -> None: ...
+
+reveal_type(X)  # N: Revealed type is "X?"
+reveal_type(inspect_sentinel)  # N: Revealed type is "def (arg: typing_extensions.Sentinel)"
+inspect_sentinel(arg=X)
+
+[builtins fixtures/tuple.pyi]