Refine description for rake test tasks.

Before:
    $ rake -T test
    rake test_test  # run all mruby tests

After:
    $ rake -T test
    rake test  # run all mruby tests

Rakefile

@@ -118,6 +118,7 @@ task :all => depfiles do
 end
 
 desc "run all mruby tests"
+task :test
 MRuby.each_target do
   next unless test_enabled?