assert_turbo_stream: accept a count

This PR allows you to provide a `count` keyword argument to `assert_turbo_stream`. The argument is just passed on to `assert_select`.

For example, if you expect a controller to remove 4 elements, you could do:

```ruby
assert_turbo_stream action: :remove, count: 4
```

lib/turbo/test_assertions.rb

@@ -7,13 +7,13 @@ module TestAssertions
       delegate :dom_id, :dom_class, to: ActionView::RecordIdentifier
     end
 
-    def assert_turbo_stream(action:, target: nil, targets: nil, status: :ok, &block)
+    def assert_turbo_stream(action:, target: nil, targets: nil, status: :ok, count: 1, &block)
       assert_response status
       assert_equal Mime[:turbo_stream], response.media_type
       selector =  %(turbo-stream[action="#{action}"])
       selector << %([target="#{target.respond_to?(:to_key) ? dom_id(target) : target}"]) if target
       selector << %([targets="#{targets}"]) if targets
-      assert_select selector, count: 1, &block
+      assert_select selector, count: count, &block
     end
 
     def assert_no_turbo_stream(action:, target: nil, targets: nil)

test/streams/streams_controller_test.rb

@@ -37,6 +37,7 @@ class Turbo::StreamsControllerTest < ActionDispatch::IntegrationTest
 
     patch message_path(id: 1), as: :turbo_stream
 
+    assert_turbo_stream action: :replace, count: 4
     assert_turbo_stream action: :replace, targets: "#message_4" do
       assert_select 'template', 'Something fourth'
     end