Implement thread retval for 1.9's Open3.popen3. JRUBY-6409

It turns out that the thread value returned from 1.9's popen3 is
actually easy to support by simply calling java.lang.Process's
waitFor method. This is, in fact, much more reliable than a normal
popen + waitpid due to our racing the JVM to the waitpid call.

There's still issues with the interactivity of the streams (on
some JVMs/versions) and overall JRuby process management, but this
patch should allow popen3 to work properly.

Unexcluded some popen3 tests that pass now, too (yay!)