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.

added Waiting for review label

LGTM

approved this merge request

merged

mentioned in commit 6a3eedef

removed Waiting for review label

changed milestone to %3.7.1