DRY in assert_broadcasts

Test `assert_no_broadcasts` failure

actioncable/lib/action_cable/test_helper.rb

@@ -47,11 +47,12 @@ module ActionCable
         original_count = broadcasts_size(stream)
         yield
         new_count = broadcasts_size(stream)
-        assert_equal number, new_count - original_count, "#{number} broadcasts to #{stream} expected, but #{new_count - original_count} were sent"
+        actual_count = new_count - original_count
       else
         actual_count = broadcasts_size(stream)
-        assert_equal number, actual_count, "#{number} broadcasts to #{stream} expected, but #{actual_count} were sent"
       end
+
+      assert_equal number, actual_count, "#{number} broadcasts to #{stream} expected, but #{actual_count} were sent"
     end
 
     # Asserts that no messages have been sent to the stream.

actioncable/test/test_helper_test.rb

@@ -62,6 +62,16 @@ class TransmissionsTest < ActionCable::TestCase
 
     assert_match(/1 .* but 2/, error.message)
   end
+
+  def test_assert_no_broadcasts_failure
+    error = assert_raises Minitest::Assertion do
+      assert_no_broadcasts "test" do
+        ActionCable.server.broadcast "test", "hello"
+      end
+    end
+
+    assert_match(/0 .* but 1/, error.message)
+  end
 end
 
 class TransmitedDataTest < ActionCable::TestCase