Make drain plugin drain spec deterministic

spec/unit/lib/delayed_job/local_worker_drain_plugin_spec.rb

@@ -23,19 +23,37 @@
     after { Delayed::Worker.sleep_delay = Delayed::Worker::DEFAULT_SLEEP_DELAY }
 
     it 'works off all remaining jobs in the queue before exiting' do
-      work_off_calls = Queue.new
+      work_off_count = 0
+      worker_started = Queue.new
+      resume_worker = Queue.new
       allow(worker).to receive(:work_off) do
-        work_off_calls.push(:called)
-        work_off_calls.size < 3 ? [1, 0] : [0, 0]
+        work_off_count += 1
+        if work_off_count == 1
+          worker_started.push(:yes)
+          # Block after the first call until the test has confirmed the TERM handler ran,
+          # so the number of work_off calls does not depend on signal-delivery timing.
+          resume_worker.pop
+        end
+        work_off_count < 3 ? [1, 0] : [0, 0]
       end
 
       worker_thread = Thread.new { worker.start }
-      work_off_calls.pop # wait until worker has started
+      worker_started.pop # wait until worker has started
+
       Process.kill('TERM', Process.pid)
+      # Wait (max. 1s) until the drain TERM handler has actually flipped the flag.
+      deadline = Process.clock_gettime(Process::CLOCK_MONOTONIC) + 1
+      until Delayed::Worker.exit_on_complete
+        raise 'Timed out waiting for Delayed::Worker.exit_on_complete' if Process.clock_gettime(Process::CLOCK_MONOTONIC) >= deadline
+
+        sleep 0.01
+      end
+      resume_worker.push(:now)
+
       worker_thread.join(5)
 
       expect(worker_thread.alive?).to be(false)
-      expect(work_off_calls.size).to eq(3)
+      expect(work_off_count).to eq(3)
     end
   end
 end