Add a button to the UI that sends a test command over Redis

[thomas-daniels]

Currently there is no way to test in advance if a UI command is going to reach the processor successfully, without actually performing an action that has some meaning. If Redis runs fine and the UI and Processor connect to it without errors, it's very unlikely that something goes wrong. However, it feels quite uncomfortable that the first command you realistically send from the UI is the "Start" command, and at that moment you really don't want any surprises with Redis or the Processor. A test command button enables testing this long before the start of the run.