[red-knot] Demote the negation_reverses_subtype_order test back to flaky (astral-sh#16951)

Fixes astral-sh#16913. See my analysis in the issue for the rationale

Author: AlexWaygood

crates/red_knot_python_semantic/src/types/property_tests.rs

@@ -103,12 +103,6 @@ mod stable {
         forall types s, t. s.is_subtype_of(db, t) && t.is_subtype_of(db, s) => s.is_equivalent_to(db, t)
     );
 
-    // If `S <: T`, then `~T <: ~S`.
-    type_property_test!(
-        negation_reverses_subtype_order, db,
-        forall types s, t. s.is_subtype_of(db, t) => t.negate(db).is_subtype_of(db, s.negate(db))
-    );
-
     // `T` is not disjoint from itself, unless `T` is `Never`.
     type_property_test!(
         disjoint_from_is_irreflexive, db,
@@ -286,4 +280,19 @@ mod flaky {
         forall types s, t.
             !s.is_disjoint_from(db, union(db, [s, t])) && !t.is_disjoint_from(db, union(db, [s, t]))
     );
+
+    // If `S <: T`, then `~T <: ~S`.
+    //
+    // DO NOT STABILISE this test until the mdtests here pass:
+    // https://github.com/astral-sh/ruff/blob/2711e08eb8eb38d1ce323aae0517fede371cba15/crates/red_knot_python_semantic/resources/mdtest/type_properties/is_subtype_of.md?plain=1#L276-L315
+    //
+    // This test has flakes relating to those subtyping and simplification tests
+    // (see https://github.com/astral-sh/ruff/issues/16913), but it is hard to
+    // reliably trigger the flakes when running this test manually as the flakes
+    // occur very rarely (even running the test with several million seeds does
+    // not always reliably reproduce the flake).
+    type_property_test!(
+        negation_reverses_subtype_order, db,
+        forall types s, t. s.is_subtype_of(db, t) => t.negate(db).is_subtype_of(db, s.negate(db))
+    );
 }