Testfw sometimes stalls

[agren]

Testfw sometimes stalls (seen using main d2c7fb0). It's noticed by that the client no longer receive data on the CDC port.
Once stalled, there will be no more output until the device is power cycled and the tests are started again.

It does not seem to tied to a specific test. I've seen the stall occur both while the UDS is printed and while numbers from the TRNG is printed.