kernel/timeout: fix k_timer_remaining_get() when tickless

In some circumstances (e.g., a tickless kernel), k_timer_remaining_get()
would not account for time passed that didn't involve clock interrupts.
This adds a simple fix for that, and adds a test case.  In addition, the
return value of k_timer_remaining_get() is clamped at 0 in the case of
overdue timers and the API description is adjusted to reflect this.

Fixes: #13353

Signed-off-by: Charles E. Youse <charles.youse@intel.com>