This race test in MRI's test_io can't be made to reliably raise
only IOError in the threads for a few reasons:

* The thread may not be at the gets call by the time the stream
  starts to be closed in the main thread, causing the gets to
  produce EBADF instead of IOError.
* Even if we wait until the thread claims to be asleep, that
  state change is not atomic and so we may proceed to close the
  stream before the gets is actually blocking on it, again
  producing EBADF

This small test tweak along with improvements to the blocking
thread logic allows the test to pass.

We have Random::Formatter from securerandom.rb, but it appears
MRI moved into core class Random at some point. Changes here need
to make that move and incorporate other changes we have not kept
up with.