fix(slack): upload test results to message

[macabu]

I noticed this error in the logs:

"error while uploading output to <ts> in slack channel <redacted>: file.upload.v2: filename cannot be empty"

Looking at Slack docs https://docs.slack.dev/messaging/working-with-files/#uploading_files, it requires both file name + file size in bytes as arguments.

[zerok]

Thank you 😄