Close TestStore. (#1655)

(cherry picked from commit aefb996ef0de7819f7317ca42af29ef65c9b2735)

Author: mbrandonw

Sources/ComposableArchitecture/TestStore.swift

@@ -426,7 +426,7 @@ import XCTestDynamicOverlay
 /// [merowing.info]: https://www.merowing.info
 /// [exhaustive-testing-in-tca]: https://www.merowing.info/exhaustive-testing-in-tca/
 /// [Composable-Architecture-at-Scale]: https://vimeo.com/751173570
-open class TestStore<State, Action, ScopedState, ScopedAction, Environment> {
+public final class TestStore<State, Action, ScopedState, ScopedAction, Environment> {
 
   /// The current dependencies of the test store.
   ///