Annotate on-disk reports with whether they've been successfully uploaded to the collector
When OONI finishes a test, it generates a report and a) writes it to disk and b) uploads it to the collector.
Currently, if it fails to upload it to the collector after three tries, it exits and discards all knowledge about whether it succeeded.
Instead, we should save-to-disk the result of the upload, so later we are able to gather and publish past reports that didn't make it to the collector.