Add support for running individual tests.

[gaby]

Currently the cli by default will run each test 10 for multiple data sizes. If your connection is slow, lets say 25Mbps the test can take 1-2 minutes to run.

It would be useful to be able to specify which to test run via cli: --test 1MB, or --test 10MB.

Another thing that would help is allowing less than 4 runs.

[gaby]

Closing this, I didnt realize the --max-payload-size flag.

[gaby]

Re-opening since even with --max-payload-size, the cli will run all previous tests.

If you specify 10m, it still runs the 100kb, 1m tests