[red-knot] Add tests asserting that KnownClass::to_instance() doesn't unexpectedly fallback to Type::Unknown with full typeshed stubs (#16608)

## Summary

One of the motivations in #16428
for panicking when the `test` or `debug_assertions` features are enabled
and a lookup of a `KnownClass` fails is that we've had some latent bugs
in our code where certain variants have been silently falling back to
`Unknown` in every typeshed lookup without us realising. But that in
itself isn't a great motivation for panicking in
`KnownClass::to_instance()`, since we can fairly easily add some tests
that assert that we don't unexpectedly fallback to `Unknown` for any
`KnownClass` variant. This PR adds those tests.

## Test Plan

`cargo test -p red_knot_python_semantic`

Author: AlexWaygood

crates/red_knot_python_semantic/src/types/class.rs

@@ -995,11 +995,10 @@ impl<'db> KnownClass {
             | Self::MethodWrapperType
             | Self::WrapperDescriptorType => KnownModule::Types,
             Self::NoneType => KnownModule::Typeshed,
-            Self::SpecialForm
-            | Self::TypeVar
-            | Self::TypeAliasType
-            | Self::StdlibAlias
-            | Self::SupportsIndex => KnownModule::Typing,
+            Self::SpecialForm | Self::TypeVar | Self::StdlibAlias | Self::SupportsIndex => {
+                KnownModule::Typing
+            }
+            Self::TypeAliasType => KnownModule::TypingExtensions,
             Self::NoDefaultType => {
                 let python_version = Program::get(db).python_version(db);
 
@@ -1603,6 +1602,7 @@ mod tests {
     use super::*;
     use crate::db::tests::setup_db;
     use crate::module_resolver::resolve_module;
+    use salsa::Setter;
     use strum::IntoEnumIterator;
 
     #[test]
@@ -1619,4 +1619,44 @@ mod tests {
             );
         }
     }
+
+    #[test]
+    fn known_class_doesnt_fallback_to_unknown_unexpectedly_on_latest_version() {
+        let mut db = setup_db();
+
+        Program::get(&db)
+            .set_python_version(&mut db)
+            .to(PythonVersion::latest());
+
+        for class in KnownClass::iter() {
+            assert_ne!(
+                class.to_instance(&db),
+                Type::unknown(),
+                "Unexpectedly fell back to `Unknown` for `{class:?}`"
+            );
+        }
+    }
+
+    #[test]
+    fn known_class_doesnt_fallback_to_unknown_unexpectedly_on_low_python_version() {
+        let mut db = setup_db();
+
+        for class in KnownClass::iter() {
+            let version_added = match class {
+                KnownClass::BaseExceptionGroup => PythonVersion::PY311,
+                KnownClass::GenericAlias => PythonVersion::PY39,
+                _ => PythonVersion::PY37,
+            };
+
+            Program::get(&db)
+                .set_python_version(&mut db)
+                .to(version_added);
+
+            assert_ne!(
+                class.to_instance(&db),
+                Type::unknown(),
+                "Unexpectedly fell back to `Unknown` for `{class:?}` on Python {version_added}"
+            );
+        }
+    }
 }