Fix reentrant mutext spec and make it run on all rubies

spec/rspec/support/reentrant_mutex_spec.rb

@@ -28,27 +28,26 @@
     order.join_all
   end
 
-  if RUBY_VERSION >= '3.0'
-    it 'waits when trying to lock from another Fiber' do
-      mutex.synchronize do
-        ready = false
-        f = Fiber.new do
-          expect {
-            ready = true
-            mutex.send(:enter)
-            raise 'should reach here: mutex is already locked on different Fiber'
-          }.to raise_error(Exception, 'waited correctly')
+  it 'waits when trying to lock from another Fiber' do
+    mutex.synchronize do
+      called = false
+      ready = false
+
+      f =
+        Fiber.new do
+          ready = true
+          mutex.send(:enter)
+          expect(called).to eq true
         end
 
-        main_thread = Thread.current
+      main_thread = Thread.current
 
-        t = Thread.new do
-          Thread.pass until ready && main_thread.stop?
-          main_thread.raise Exception, 'waited correctly'
-        end
-        f.resume
-        t.join
+      t = Thread.new do
+        Thread.pass until ready && main_thread.stop?
+        called = true
       end
+      f.resume
+      t.join
     end
   end
 end