Test: index dump on test failure

This changes adds the ability for a test to create an index dump if a search request fails the test. This can be helpful to be able to analyse the index state after the test has failed
Sorry, I completely missed the name of the method before!  However, since this is the repo name, which is essentially the base path for the snapshot, why not name it something related to the test, rather than random? This would be easier to figure out what you could delete, rather than having to remember the uuids for repos that were created from past test runs.