tests: logging: log_msg: Fix test after mpsc_pbuf changes

Test started to fail after updated in mpsc_pbuf. It was failing for
two reasons:
- Expected capacity was not updated after change in mpsc_pbuf which now
has full capacity (does not use extra byte for empty vs full
distinction).
- Messages were claimed without freeing. It was faulty even before the
update since mpsc_pbuf is single consumer so only one message can be
claimed at a time. However, test was passing before unexpectedly.

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