Remove backtrace set_hook from test-integration

[jbesraa]

While the panic trace can be annoying if the error originates from unhandled message(s), it is quite useful if it is a different kind of error. Initially I thought removing the stack trace would be cleaner and would result in better logs, but in practice any error that occurs in testing is currently not printed, unless it is an unhandled message.

This should prevent cases like #1374