Thanks for the question, @BastianKusserow. swift-testing already performs automatic test discovery at runtime by walking emitted Swift metadata sections. 
SE-0385 was returned for revision and is, in part, superseded by macros as a language feature.
@kubamracek and his colleagues are investigating future language features that would allow emitting custom metadata in a format that swift-testing could adopt. For more information about that effort, see Kuba's first pitch here and his second pitch here.