improvement: add etc_group support for centos and add integration test

lib/resources/etc_group.rb

@@ -37,7 +37,7 @@ class EtcGroup < Inspec.resource(1)
 
     # skip resource if it is not supported on current OS
     return skip_resource 'The `etc_group` resource is not supported on your OS.' \
-    unless %w{ubuntu debian redhat fedora arch darwin freebsd}.include?(inspec.os[:family])
+    unless %w{ubuntu debian redhat fedora centos arch darwin freebsd}.include?(inspec.os[:family])
   end
 
   def groups(filter = nil)

test/integration/test/integration/default/etc_group.rb

@@ -0,0 +1,13 @@
+# encoding: utf-8
+
+if os.unix?
+  describe etc_group do
+    its('gids') { should_not contain_duplicates }
+    its('groups') { should include 'root' }
+    its('users') { should include 'root' }
+  end
+
+  describe etc_group.where(name: 'root') do
+    its('users') { should include 'root' }
+  end
+end