tests/schedule_api: Correct tick/ms math for non-divisible clocks

Adjusting the tick alignment of this test caused it to start breaking
on nRF5 platforms, which use a 32768 Hz clock that doesn't divide
evenly into the millisecond precision used by the test.  The "half
slice" math ended up being wrong by a bit.

Convert to ticks first before computing the cycle delay needed.

Fixes #29705

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>