[FIXUP] Add test for unsetting explicit global

test/unit/iteration_test.rb

@@ -298,6 +298,23 @@ def test_per_class_max_job_runtime_with_global_set
     end
   end
 
+  def test_max_job_runtime_cannot_unset_global
+    with_global_max_job_runtime(30.seconds) do
+      klass = Class.new(ActiveJob::Base) do
+        include JobIteration::Iteration
+      end
+
+      error = assert_raises(ArgumentError) do
+        klass.max_job_runtime = nil
+      end
+
+      assert_equal(
+        "max_job_runtime may only decrease; #{klass} tried to increase it from 30 seconds to nil (no limit)",
+        error.message,
+      )
+    end
+  end
+
   def test_max_job_runtime_cannot_be_higher_than_global
     with_global_max_job_runtime(30.seconds) do
       klass = Class.new(ActiveJob::Base) do