Swift bug workaround for TestStore completion

[stephencelis]

Due to a Swift bug, stores and test stores initialized with type inference are broken when it comes to autocomplete:

swiftlang/swift#72399

For example, this test store cannot be completed from:

let store = TestStore(initialState: Feature.State()) {
  Feature()
}

While this can affect regular stores, as well, it is less likely outside of root stores, since stores are typically declared in views and view controllers as properties, and inference isn't required:

let store: StoreOf<Feature>

While the user can employ a workaround in their project by making the types very explicit:

-let store = TestStore(initialState: Feature.State()) {
+let store: TestStoreOf<Feature> = TestStore(initialState: Feature.State()) {
   Feature()
 }

This PR takes the approach of removing the builder context from TestStore.init, since the issue is more common in tests, and because composing a reducer for a test store is less common.

[stephencelis]

We unfortunately can't add a deprecated, disfavored overload of this initializer.

Invalid redeclaration of 'init(initialState:reducer:withDependencies:file:line:)'