tests: timer_behavior: don't fail the test with timer wrap-arounds

If the timer driver only implements sys_clock_cycle_get_32() (meaning
CONFIG_TIMER_HAS_64BIT_CYCLE_COUNTER=n) and the hardware clock is high
enough then the reported cycle count may wrap an uint32_t during the
test. This makes validating the total test duration pointless as it
cannot be measured. Just print a warning instead of failing the test
in that case.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>