Use gotestsum instead of go-junit-report

Start using gotestsum as per the gitlab test report docs.

• gotestsum docs
• Gitlab docs

There are quite a few benefits listed here gitlab!66787 (merged)

The main ones for us are:

• This project runs on the actual json output of go test so is less likely to have parsing issues
• It outputs a failure summary so we don't need to constantly search logs for failures

In addition I believe this will allow us to start using testing.T.Log for our test logging instead of using null logrus loggers. This would mean logging only with -v or on failure. Previously go-junit-report forced us to always use verbose mode to get the output it could parse, but this also meant all the logging all the time.