Fix flaky test.

Author: ioquatix

test/fiber/scheduler.rb

@@ -497,7 +497,7 @@ def io_write(io, buffer, length, offset)
     fd = io.fileno
     str = buffer.get_string
     __io_ops__ << [:io_write, fd, str]
-    Fiber.blocking { buffer.write(IO.for_fd(fd), 0, offset) }
+    Fiber.blocking { buffer.write(io, 0, offset) }
   end
 end
 

test/fiber/test_scheduler.rb

@@ -287,7 +287,6 @@ def test_post_fork_fiber_blocking
   end
 
   def test_io_write_on_flush
-    omit "skip this test because it makes CI fragile"
     begin
       fn = File.join(Dir.tmpdir, "ruby_test_io_write_on_flush_#{SecureRandom.hex}")
       write_fd = nil