like r64991, until we get helpful error logs on worker death or the
unstability is fixed.

Since mspec worker randomly dies on MinGW, maybe it should have an
option to retry worker death.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65050 b2dd03c8-39d4-4d8f-98ff-823fe69b080e