The macro with name `N` is used by the MSP430 CPU macros and
triggers a compilation error if this example is build for that
platform. So this commit renames the repeat counter limit from
`N` to `REAPEAT`.
Instead of only testing for `ZTIMER_MSEC`, iterate over a list of
clocks and run the test for each. The reasoning is that this test
does not only test `ztimer_periodic`, but also that the used clock
backend operates correctly in a typical use case.
