[ruby-core:42808] JRuby now running MRI's test suite

I just wanted to update ruby-core that JRuby is now running MRI's
tests as part of our regular suite.

We worked with Ryan Davis to design an extension to minitest called
"excludes" which allows us to exclude certain class#method
combinations from test runs. This has allowed us to start running
MRI's tests and maintain a high-water mark of passing tests.

minitest-excludes is available as a gem. I have a few pending fixes
that should go into the next release that we needed to fully run MRI's
suite, but it has been working very well so far.

Currently we are just running test/ruby/** and most of the native
extension tests. The others largely test stdlib that we share in
common with MRI. After building JRuby, "rake test:mri19" will run the
MRI 1.9.3 suite.

JRuby's current excludes are here:
https://github.com/jruby/jruby/tree/master/test/externals/ruby1.9/excludes

We periodically sync 1.9.3's tests here:
https://github.com/jruby/jruby/tree/master/test/externals/ruby1.9

And as always, we maintain any diffs of stdlib in our fork here:
https://github.com/jruby/ruby/tree/jruby-ruby_1_9_3

We're very happy to be able to run MRI's tests along with our other
suites. Hopefully over the coming months we'll be able to contribute
improvements to the tests.

Thank you!

- Charlie