Make JRuby tmpdir's behave like MRI's

Adapted from
https://github.com/rspec/rspec-core/blob/06cdba596eb636fa3b30408e7117168143703544/spec/spec_helper.rb#L16

Fixes a failure like:
 AppRailsLoaderTest#test_is_in_a_Rails_application_if_parent_directory_has_bin/rails_containing_ENGINE_
 PATH_and_chdirs_to_the_root_directory
 [test/app_rails_loader_test.rb:80]:
 --- expected
 +++ actual
 @@ -1 +1 @@
 -"/var/folders/9f/09q8yff53l1f0dhbc4h2m6sh0000gp/T/railties-rails-loader-test-suite20141110-29736-ed5oea/foo"
 +"/private/var/folders/9f/09q8yff53l1f0dhbc4h2m6sh0000gp/T/railties-rails-loader-test-suite20141110-29736-ed5oea/foo"