add mirror speed test (latency + transfer)

[mx1up]

Hi

the manual method described in the documentation on how to select a mirror does no longer work. It is not possible anymore to select a mirror manually and test its speed (at least I did not find it in the renewed site).

So I added a button in your program to test a mirror's speed.

Thanks for the helper program, without it it took me ages to download (and cancelled eventually ;)). Now it is downloaded in the time it took me to create this pull request.