Import specs for Fiber#[] from MRI

This removes one of our tests, and the new test is very minimal. This should be
extended, but at least it enables us to run the spec under Ruby 3.4.

core/fiber/storage_spec.rb

@@ -90,12 +90,14 @@
         key = :"#{self.class.name}#.#{self.object_id}"
         Fiber.new { Fiber[key] = 42; Fiber[key] }.resume.should == 42
       end
+    end
 
-      it "can't use invalid keys" do
-        invalid_keys = [Object.new, "Foo", 12]
-        invalid_keys.each do |key|
-          -> { Fiber[key] }.should raise_error(TypeError)
-        end
+    ruby_bug "#20978", "3.2.3"..."3.4" do
+      it "can use keys as strings" do
+        key = Object.new
+        def key.to_str; "Foo"; end
+        Fiber[key] = 42
+        Fiber["Foo"].should == 42
       end
     end