fix(rubocop): add fixes using `rubocop --safe-auto-correct`

test/integration/default/controls/backend_spec.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 describe package('nftables') do
   it { should be_installed }
 end

test/integration/default/controls/firewalld_spec.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 describe package('firewalld') do
   it { should be_installed }
 end

test/integration/default/controls/ipset_spec.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 describe package('ipset') do
   it { should be_installed }
 end