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

1 files changed, 1 insertions, 0 deletions

diff --git a/Rakefile b/Rakefile
index 20b6096f5..47da28166 100644
--- a/Rakefile
+++ b/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?