[tests] Experiment with fixing test race condition on servlet_context

Author: chadlwilson

src/spec/ruby/jruby/rack/rails_booter_spec.rb

@@ -120,10 +120,7 @@
       [:app_path, :public_path, :context].each do |name|
         JRuby::Rack.send :remove_instance_variable, :"@#{name}"
       end
-    end
-
-    after :all do
-      $servlet_context = nil
+      $servlet_context = nil if defined? $servlet_context
     end
 
     let(:railtie_class) { Class.new(Rails::Railtie) }