Add spec to cover resume a resuming fiber error

Author: bjfish

library/fiber/resume_spec.rb

@@ -10,6 +10,12 @@
       fiber2.resume
       -> { fiber2.resume }.should raise_error(FiberError)
     end
+
+    it "raises a FiberError if the Fiber attempts to resume a resuming fiber" do
+      root_fiber = Fiber.current
+      fiber1 = Fiber.new { root_fiber.resume }
+      -> { fiber1.resume }.should raise_error(FiberError, /double resume/)
+    end
   end
 
   ruby_version_is '3.0' do
@@ -19,5 +25,11 @@
       fiber2.resume.should == 10
       fiber2.resume.should == 20
     end
+
+    it "raises a FiberError if the Fiber attempts to resume a resuming fiber" do
+      root_fiber = Fiber.current
+      fiber1 = Fiber.new { root_fiber.resume }
+      -> { fiber1.resume }.should raise_error(FiberError, /attempt to resume a resuming fiber/)
+    end
   end
 end