testsuite: ztest: Do logging flush only when default thread is used

Add guard to not pend until all logs are flushed when default
logging thread is disabled. In that case, logging has no control
where logs are processed and sleeping may not lead to flushing
log data. That may result in test hanging.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>