Fix FD leaks (#752)

`PTY.spawn` with a block detaches the spawned process and leaves it
running in background even after exiting the given block.  It is the
responsibility of the caller to clean up the yielded IOs and PID.

https://github.com/ruby/ruby/actions/runs/11148759246/job/30986064044#step:13:950
```
Leaked file descriptor: Reline::Test#test_tty_amibuous_width: 9 : #<File:/dev/pts/0>
Leaked file descriptor: Reline::Test#test_tty_amibuous_width: 10 : #<File:/dev/pts/0>
Leaked file descriptor: Reline::Test#test_tty_amibuous_width: 11 : #<File:/dev/pts/1>
Leaked file descriptor: Reline::Test#test_tty_amibuous_width: 12 : #<File:/dev/pts/1>
Leaked file descriptor: Reline::Test#test_tty_amibuous_width: 13 : #<File:/dev/pts/2>
Leaked file descriptor: Reline::Test#test_tty_amibuous_width: 14 : #<File:/dev/pts/2>
```

Author: nobu

test/reline/test_reline.rb

@@ -454,6 +454,9 @@ def test_tty_amibuous_width
         loop { break if r.readpartial(1024).include?("\e[6n") }
         w.puts "hello\e[10;#{ambiguous_width + 1}Rworld"
         assert_include(r.gets, [ambiguous_width, 'helloworld'].inspect)
+      ensure
+        r.close
+        w.close
         Process.waitpid pid
       end
     end
@@ -463,6 +466,9 @@ def test_tty_amibuous_width
       loop { break if r.readpartial(1024).include?("\e[6n") }
       w.puts "hello\e[10;2Sworld"
       assert_include(r.gets, [1, "hello\e[10;2Sworld"].inspect)
+    ensure
+      r.close
+      w.close
       Process.waitpid pid
     end
   end