Check join is symmetric

Author: ilevkivskyi

test-data/unit/check-typevar-tuple.test

@@ -2594,8 +2594,11 @@ def join(x: T, y: T) -> T: ...
 def test(xs: tuple[Unpack[Ts]], xsi: tuple[int, Unpack[Ts]]) -> None:
     a: tuple[Any, ...]
     reveal_type(join(xs, a))  # N: Revealed type is "builtins.tuple[Any, ...]"
+    reveal_type(join(a, xs))  # N: Revealed type is "builtins.tuple[Any, ...]"
     aa: tuple[Unpack[tuple[Any, ...]]]
     reveal_type(join(xs, aa))  # N: Revealed type is "builtins.tuple[Any, ...]"
+    reveal_type(join(aa, xs))  # N: Revealed type is "builtins.tuple[Any, ...]"
     ai: tuple[int, Unpack[tuple[Any, ...]]]
     reveal_type(join(xsi, ai))  # N: Revealed type is "Tuple[builtins.int, Unpack[builtins.tuple[Any, ...]]]"
+    reveal_type(join(ai, xsi))  # N: Revealed type is "Tuple[builtins.int, Unpack[builtins.tuple[Any, ...]]]"
 [builtins fixtures/tuple.pyi]