I am not convinced the intermittent failures here indicate an
issue in JRuby. Rather, I suspect there's a race, since these
tests involve setting up a signal handler, signalling INT from a
thread, and joining in both main and the handler itself. However
I have been unable to come up a reason for the race, so I have
filed #4956 to track this exclusion.

An example script, which should run forever but dies:

```ruby
loop {
  Signal.trap(:INT, "DEFAULT")
  t0 = Thread.current
  t = Thread.new {Thread.pass until t0.stop?; Process.kill(:INT, $$)}

  Signal.trap :INT do
    t.join
  end

  t.join
  print '.'
}
```