(API) Allow for caller to specify the size of upload/download tests

[cirne]

https://github.com/showwin/speedtest-go/blob/856e6e6726b7fc8e8552075805ef1a92d0c26b81/speedtest/request.go#L96

It appears that server.DownloadTest and server.UploadTest use a hard coded image size (which essentially drives the bandwidth impact of these tests) to dlSizes[3] and ulSizes[3].

https://github.com/showwin/speedtest-go/blob/856e6e6726b7fc8e8552075805ef1a92d0c26b81/speedtest/request.go#L20-L23

I am using the GO API to write a simple program that periodically runs speedtest on a regular basis. I'd like to reduce the size of each download/upload test to reduce overall bandwidth consumption since it will be run on an ongoing basis. Recognizing the is likely a tradeoff between test size/bandwith impact and accuracy, but it would be great if the developer could evaluate that tradeoff rather than rely on the hardcoded values currently provided.

[r3inbowari]

Hi, @cirne. I think reduce the size of each download/upload request will not reduce overall bandwidth consumption. Because reducing the size will send more download/upload requests. The overall result will not be much different.