Fix rubocop: Style/SafeNavigation

Addressing:
test/common/testcase.rb:20:7: C: Style/SafeNavigation: Use safe navigation (&.) instead of checking if an object exists before calling the method.
      @s.destroy if @s
      ^^^^^^^^^^^^^^^^

Author: isimluk

test/common/testcase.rb

@@ -17,7 +17,7 @@ def setup
     end
 
     def cleanup
-      @s.destroy if @s
+      @s&.destroy
       Dir.chdir '../..'
       OpenSCAP.raise! if OpenSCAP.error?
       OpenSCAP.oscap_cleanup