Ruby

envutil.rb: no signal description iff possible

• test/lib/envutil.rb (FailDesc): show signal description only if
  it is available, or nothing if not killed by a signal.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50905 b2dd03c8-39d4-4d8f-98ff-823fe69b080e