tests: timer_api: fix assertion in test_timer_remaining()

This commit fixes the assertion in test_timer_remaining() that checks
whether the remaining ticks on a timer is less than or equal to half of
the timer duration after a busy wait of that time. If the timer
duration corresponds to an odd number of ticks, 1 should be added to
the upper bound given k_timer_remaining_ticks() returns
<ticks til next deadline> - <elapsed ticks>,
and <elapsed ticks> is truncated to closest integer tick count.

For example, if
    dur_ticks = 3277
    <ticks til next deadline> = 3277
    <elapsed ticks> = 1638.5 rounded to 1638

rem_ticks would be 1639, which is 1 greater than dur_ticks/2.

Fixes #25331

Signed-off-by: Vincent Wan <vincent.wan@linaro.org>