tests: kernel: timer: timer_api: reorder test_timer_remaining step

Move init_timer_data() out of k_usleep() tick alignment.
Compute rem_ticks just after busy_wait_ms() to avoid slew
due to 'now' and 'rem_ms' computations.
With slow CPU 32MHz: -2 Ticks.

Signed-off-by: Alexandre Bourdiol <alexandre.bourdiol@st.com>