Basic test for periph/rtt introduced in #15431 is incorrect

[chudov]

Description

Basic tests introduced in #15431 failed for RTT_FREQUENCY that are not a multiple of 2^n.
The test assumes RTT time frequency is 1 Hz, 1024 Hz, 16384 Hz or 32768 Hz.
This assumption is wrong for RTT on ATMega128RFR2 and ATMega256RFR2. On these MCU the RTT has frequency 62500 Hz because RTT is based on 802.15.4 MAC symbols counter.

Steps to reproduce the issue

Run periph_rtt test on any board based on Atmel ATMega*RFR2: deRFmega128, deRFmega256, atmega256rfr2-xpro, avr-rss2.

Expected results

Test passes for platforms that use RTT_FREQUENCY that is not a multiple of 2^n.

Actual results

RIOT RTT low-level driver test<\n>
RTT configuration:<\n>
RTT_MAX_VALUE: 0x0000ffff<\n>
RTT_FREQUENCY: 62500<\n>
<\n>
Testing the tick conversion<\n>
Trying to convert 1 to seconds and back<\n>
Trying to convert 256 to seconds and back<\n>
Trying to convert 65536 to seconds and back<\n>
error: TICK conversion not working as expected

Versions

RIOT Version: 2020.10
Compiler version irrelated to the issue