Add tests for AJ events

Because events are documented but not tested.

activejob/test/cases/instrumentation_test.rb

@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+require "helper"
+require "jobs/hello_job"
+require "jobs/retry_job"
+
+class InstrumentationTest < ActiveSupport::TestCase
+  include ActiveJob::TestHelper
+
+  setup do
+    JobBuffer.clear
+  end
+
+  test "perform_now emits perform events" do
+    events = subscribed(/perform.*\.active_job/) { HelloJob.perform_now("World!") }
+    assert_equal 2, events.size
+    assert_equal "perform_start.active_job", events[0].first
+    assert_equal "perform.active_job", events[1].first
+  end
+
+  test "perform_later emits an enqueue event" do
+    events = subscribed("enqueue.active_job") { HelloJob.perform_later("World!") }
+    assert_equal 1, events.size
+  end
+
+  test "retry emits an enqueue retry event" do
+    events = subscribed("enqueue_retry.active_job") do
+      perform_enqueued_jobs { RetryJob.perform_later("DefaultsError", 2) }
+    end
+    assert_equal 1, events.size
+  end
+
+  test "retry exhaustion emits a retry_stopped event" do
+    events = subscribed("retry_stopped.active_job") do
+      perform_enqueued_jobs { RetryJob.perform_later("CustomCatchError", 6) }
+    end
+    assert_equal 1, events.size
+  end
+
+  test "discard emits a discard event" do
+    events = subscribed("discard.active_job") do
+      perform_enqueued_jobs { RetryJob.perform_later("DiscardableError", 2) }
+    end
+    assert_equal 1, events.size
+  end
+
+  def subscribed(name, &block)
+    [].tap do |events|
+      ActiveSupport::Notifications.subscribed(-> (*args) { events << args }, name, &block)
+    end
+  end
+end