tests: kernel: timer: timer_behavior: Adjust TIMER_TEST_SAMPLES

Adjust default number of test samples which is based on SRAM size.
Test is using 8*TIMER_TEST_SAMPLES and with previous defaults for
the device with 64k RAM it was using 56k of test data leaving only
8k RAM and that was easily not enough. Adjust conditions to
take less samples when SRAM_SIZE is equal to the threshold.

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