tests: logging: log_api: Fix test_log_arguments case

Test had by accident an early return and after removal case was failing
in certain configurations. It was failing because set of long messages
was not fitting into the logging buffer used in the test. Fixed the
test by adding more frequent processing between the messages.

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