Fix/experiment type dict access

While writing !734 (merged) I misread the response from mymdc and then wrote the tests wrong, the field data_groups_repositories in get_run_by_id is a list, but in all responses I saw it was a list of one element, which I must have just skimmed over...

@ahmedk noticed this after deployment, so I did a hot-fix, everything worked correctly afterwards.

PR adjusts the tests case to correctly match mymdc and fixes the code.