periph_rtt: ensure reliable operation even for zero timeout

[mguetschow]

Quoting @maribu from #21743 (comment):

(...) Mid term we should have the RTT driver fixed to reliably trigger on a timeout as low as 0.

For periph_timer we have tests/periph/timer_short_relative_set as test for this exact bug. I guess we need the same for periph_rtt.

#21743 gives an example of a misconfigured RTT driver, which could have been catched with such as test.