Added `deliver_enqueued_emails` to `ActionMailer::TestHelper`. This method delivers all enqueued email jobs.

Example:

```ruby
def test_deliver_enqueued_emails
  deliver_enqueued_emails do
    ContactMailer.welcome.deliver_later
    end
  assert_emails 1
end
```

actionmailer/CHANGELOG.md

@@ -1,3 +1,17 @@
+*   Added `deliver_enqueued_emails` to `ActionMailer::TestHelper`. This method
+    delivers all enqueued email jobs.
+    
+    Example:
+
+    ```ruby
+    def test_deliver_enqueued_emails
+      deliver_enqueued_emails do
+        ContactMailer.welcome.deliver_later
+      end
+      assert_emails 1
+    end
+    ```
+
 *   The `deliver_later_queue_name` used by the default mailer job can now be
     configured on a per-mailer basis. Previously this was only configurable
     for all mailers via `ActionMailer::Base`.

actionmailer/lib/action_mailer/test_helper.rb

@@ -50,7 +50,7 @@ module ActionMailer
     def assert_emails(number, &block)
       if block_given?
         original_count = ActionMailer::Base.deliveries.size
-        perform_enqueued_jobs(only: ->(job) { delivery_job_filter(job) }, &block)
+        deliver_enqueued_emails(&block)
         new_count = ActionMailer::Base.deliveries.size
         diff = new_count - original_count
         assert_equal number, diff, "#{number} emails expected, but #{diff} were sent"
@@ -208,6 +208,45 @@ module ActionMailer
       assert_enqueued_emails 0, &block
     end
 
+    # Delivers all enqueued emails. If a block is given, delivers all of the emails
+    # that were enqueued throughout the duration of the block. If a block is
+    # not given, delivers all the enqueued emails up to this point in the test.
+    #
+    #   def test_deliver_enqueued_emails
+    #     deliver_enqueued_emails do
+    #       ContactMailer.welcome.deliver_later
+    #     end
+    #     assert_emails 1
+    #   end
+    #
+    #   def test_deliver_enqueued_emails_without_block
+    #     ContactMailer.welcome.deliver_later
+    #
+    #     deliver_enqueued_emails
+    #
+    #     assert_emails 1
+    #
+    #   end
+    #
+    # If the +:queue+ option is specified,
+    # then only the emails(s) enqueued to a specific queue will be performed.
+    #
+    #   def test_deliver_enqueued_emails_with_queue
+    #     deliver_enqueued_emails queue: :external_mailers do
+    #       CustomerMailer.deliver_later_queue_name = :external_mailers
+    #       CustomerMailer.welcome.deliver_later # will be performed
+    #       EmployeeMailer.deliver_later_queue_name = :internal_mailers
+    #       EmployeeMailer.welcome.deliver_later # will not be performed
+    #     end
+    #     assert_emails 1
+    #   end
+    #
+    # If the +:at+ option is specified, then only delivers emails enqueued to deliver
+    # immediately or before the given time
+    def deliver_enqueued_emails(queue: nil, at: nil, &block)
+      perform_enqueued_jobs(only: ->(job) { delivery_job_filter(job) }, queue: queue, at: at, &block)
+    end
+
     private
       def delivery_job_filter(job)
         job_class = job.is_a?(Hash) ? job.fetch(:job) : job.class

actionmailer/test/test_helper_test.rb

@@ -487,6 +487,68 @@ class TestHelperMailerTest < ActionMailer::TestCase
       end
     end
   end
+
+  def test_deliver_enqueued_emails_with_no_block
+    assert_nothing_raised do
+      silence_stream($stdout) do
+        TestHelperMailer.test.deliver_later
+        deliver_enqueued_emails
+      end
+    end
+
+    assert_emails(1)
+  end
+
+  def test_deliver_enqueued_emails_with_a_block
+    assert_nothing_raised do
+      deliver_enqueued_emails do
+        silence_stream($stdout) do
+          TestHelperMailer.test.deliver_later
+        end
+      end
+    end
+
+    assert_emails(1)
+  end
+
+  def test_deliver_enqueued_emails_with_custom_delivery_job
+    assert_nothing_raised do
+      deliver_enqueued_emails do
+        silence_stream($stdout) do
+          CustomDeliveryMailer.test.deliver_later
+        end
+      end
+    end
+
+    assert_emails(1)
+  end
+
+  def test_deliver_enqueued_emails_with_custom_queue
+    assert_nothing_raised do
+      deliver_enqueued_emails(queue: CustomQueueMailer.deliver_later_queue_name) do
+        silence_stream($stdout) do
+          TestHelperMailer.test.deliver_later
+          CustomQueueMailer.test.deliver_later
+        end
+      end
+    end
+
+    assert_emails(1)
+    assert_enqueued_email_with(TestHelperMailer, :test)
+  end
+
+  def test_deliver_enqueued_emails_with_at
+    assert_nothing_raised do
+      deliver_enqueued_emails(at: 1.hour.from_now) do
+        silence_stream($stdout) do
+          TestHelperMailer.test.deliver_later
+          TestHelperMailer.test.deliver_later(wait: 2.hours)
+        end
+      end
+    end
+
+    assert_emails(1)
+  end
 end
 
 class AnotherTestHelperMailerTest < ActionMailer::TestCase