Allow this test to retry 10 times

The fileno mapping is managed globally for a given JRuby runtime
and during testing there may be other threads still cleaning up
and removing filenos from the mapping. We give this test ten tries
to have the starting count and ending count match.

This should help avoid intermittent failures like the following:

```
Failure: test_rubyio_fileno_mapping_leak(TestIO)
/home/runner/work/jruby/jruby/test/jruby/test_io.rb:527:in `test_rubyio_fileno_mapping_leak'
     524:     io = org.jruby.RubyIO.new(JRuby.runtime, java.io.ByteArrayOutputStream.new)
     525:
     526:     open_io_count = fileno_util.number_of_wrappers
  => 527:     assert_equal(starting_count + 1, open_io_count)
     528:
     529:     io.close
     530:     closed_io_count = fileno_util.number_of_wrappers
```

test/jruby/test_io.rb

@@ -518,16 +518,30 @@ def test_stringio_gets_separator
   # JRUBY-6137
   def test_rubyio_fileno_mapping_leak
     fileno_util = JRuby.runtime.fileno_util
-    starting_count = fileno_util.number_of_wrappers
 
-    # use a non-channel stream to ensure we use our mapping
-    io = org.jruby.RubyIO.new(JRuby.runtime, java.io.ByteArrayOutputStream.new)
+    # other test threads may still be cleaning up filenos, so we give a few rounds for this to settle
+    count = 0
+    while count < 10
+      starting_count = fileno_util.number_of_wrappers
 
-    open_io_count = fileno_util.number_of_wrappers
-    assert_equal(starting_count + 1, open_io_count)
+      # use a non-channel stream to ensure we use our mapping
+      io = org.jruby.RubyIO.new(JRuby.runtime, java.io.ByteArrayOutputStream.new)
 
-    io.close
-    closed_io_count = fileno_util.number_of_wrappers
+      open_io_count = fileno_util.number_of_wrappers
+
+      io.close
+      closed_io_count = fileno_util.number_of_wrappers
+
+      if starting_count == closed_io_count
+        break
+      end
+
+      # either leaking or other threads are opening and closing; pause and try again
+      Thread.pass
+    end
+
+    # proceed to assertions
+    assert_equal(starting_count + 1, open_io_count)
     assert_equal(starting_count, closed_io_count)
   end if RUBY_ENGINE == 'jruby'