Return the total amount of downloaded/uploaded bytes

[berrnd]

Resubmit of #608

Hi and first thanks a lot for LibreSpeed!

I wanted to show how much data volume the upload / download test used. So I've added 2 properties to the speedtest_worker.js status data object:

• dlBytes: Total number of bytes downloaded
• ulBytes: Total number of bytes uploaded

This also includes data during dlGraceTime / ulGraceTime.

Could look like that (a live example is available here):

https://github.com/librespeed/speedtest/assets/2161815/4f761788-c76a-41cf-a8c1-454ee80d9268