skip when group name is not found

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63702 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/library/etc/struct_group_spec.rb

@@ -3,12 +3,16 @@ require 'etc'
 
 describe "Struct::Group" do
   platform_is_not :windows do
+    grpname = IO.popen(%w'id -gn', err: IO::NULL, &:read)
+    next unless $?.success?
+    grpname.chomp!
+
     before :all do
       @g = Etc.getgrgid(`id -g`.strip.to_i)
     end
 
     it "returns group name" do
-      @g.name.should == `id -gn`.strip
+      @g.name.should == grpname
     end
 
     it "returns group password" do