ci: gem-test: ensure we don't use local files

[skip ci]

despite best efforts to date, `rake test` would still load the local
`lib` files, likely because hoe+minitest runs the test command as
`ruby -Ilib:test:.`

related to #1971

concourse/tasks/gem-test/gem-install-and-test.sh

@@ -23,6 +23,8 @@ pushd nokogiri
   bundle install --local || bundle install
   bundle info nokogiri
 
-  bundle exec rake test
+  bundle exec rake test:cmd > run-test
+  rm -rf lib ext # ensure we can't use the local files
+  bundle exec bash run-test
 
 popd