Restore `rake TEST=test/unit/foo_test.rb` support.

Add your own test tasks as dependencies on the `test:run` task instead of `test` itself.

  namespace :test do
    task run: 'test:acceptance'

    Rails::SubTestTask.new acceptance: 'test:prepare' do |t|
      t.libs << 'test'
      t.pattern = 'test/acceptance/**/*_test.rb'
    end
  end

railties/lib/rails/test_unit/testing.rake

@@ -83,13 +83,17 @@ end
 task :default => :test
 
 desc 'Runs test:units, test:functionals, test:integration together (also available: test:benchmark, test:profile, test:plugins)'
-task :test => %w(test:units test:functionals test:integration)
+task :test do
+  Rake::Task[ENV['TEST'] ? 'test:single' : 'test:run'].invoke
+end
 
 namespace :test do
   task :prepare do
     # Placeholder task for other Railtie and plugins to enhance. See Active Record for an example.
   end
 
+  task :run => %w(test:units test:functionals test:integration)
+
   Rake::TestTask.new(:recent => "test:prepare") do |t|
     since = TEST_CHANGES_SINCE
     touched = FileList['test/**/*_test.rb'].select { |path| File.mtime(path) > since } +