tests: subsys: logging: log_api: Fix test for 64 bit timestamp

Test for dropping log messages was expecting certain number of
dropped messages assuming that buffer size is dividable by the
message size. That was not the case when timestamp was 64 bit.
In that case, additional message is dropped.

Modified the test to take that into account.

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