envutil.rb: avoid error on OS X 10.10

* test/ruby/envutil.rb (diagnostic_reports): Mac OS X 10.10 may
  not create plist file with crash file together.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46397 b2dd03c8-39d4-4d8f-98ff-823fe69b080e