Enable XML rspec results

XML results can be produced by rspec which can then be consumed by gitlab to give pretty results in merge requests.

https://docs.gitlab.com/ee/ci/junit_test_reports.html#ruby-example

Probably worth mentioned the normal existing logs inline are still created.

I've added code/rspec.xml to the .gitignore this is not really needed as the file is only ever generated in CI where the SPEC_OPTS is set but in case someone sets it locally for some reason they won't be messed up.