Use the test-unit-ruby-core gem for Test::Unit::CoreAssertions

test/lib/core_assertions.rb and its dependencies are now packaged in
the gem test-unit-ruby-core. Let's use it instead.

Author: rhenium

Gemfile

@@ -6,6 +6,7 @@ group :development do
   gem "rake"
   gem "rake-compiler"
   gem "test-unit", "~> 3.0", ">= 3.4.6"
+  gem "test-unit-ruby-core"
   # In the case of Ruby whose rdoc is not a default gem.
   gem "rdoc"
 end