xcvradm should deliver test status messages

[Aaron-Hartwig]

Related to oxidecomputer/hubris#1783

When we are using Medusa to test Front IO boards, it would be nice to be able to tell the LEDs what color to be over the network (as that is how the test will be administered). We should noodle on what those statuses would be and what the appropriate colors would be. Some initial thoughts:

• "no test running" = LEDs off
• "test running" = blue
• "test pass" = green
• "test fail" = red