Add "Test" button to UI to request device net speed testing

[Saxon Milton]

netsender clients can now perform a net speed test in response to a “Test” response code. We’ll need to add a “Test” button next to the other actions such as Reboot, Debug, Update etc. The device status code corresponding to a net speed test is 6.

[Alan Noble]

VidGrind does not display the correct “speed test” icon when in the new test mode.

[Saxon Milton]

This now works with the go.mod required iotsvc version bumped from 0.2.14 to 0.2.15.

[Saxon Milton]

Resolved in PR #88