spec/ruby/library/etc/: skip the specs related to group on Android

User/group system on Android seems different from normal Linux.

spec/ruby/library/etc/getgrnam_spec.rb

@@ -11,7 +11,7 @@ platform_is :windows do
   end
 end
 
-platform_is_not :windows do
+platform_is_not :windows, :android do
   describe "Etc.getgrnam" do
     it "returns a Etc::Group struct instance for the given group" do
       gr_name = Etc.getgrent.name

spec/ruby/library/etc/group_spec.rb

@@ -5,7 +5,7 @@ require 'etc'
 describe "Etc.group" do
   it_behaves_like :etc_on_windows, :group
 
-  platform_is_not :windows do
+  platform_is_not :windows, :android do
     it "returns a Etc::Group struct" do
       group = Etc.group
       begin