These tests need to be tested on xtimer, and if using ztimer_xtimer_compat
the full api will not be provided, therefore also blacklist BOARDs
that will en up selecting ztimer by default.
Blacklist native for the xtimer tests that have timing issues.
This also enables running the test on `nrf52dk` as I think it was
forgotten when adding the support.
The test randomly fails for unrelated PRs. Expecting real-time timing on
native is not really possible. Maybe it could be fixed but it currently
triggers many false positive.
- adapt code to use public, typesafe API calls
- add some statistical output
- minor code enhancements and doc corrections
- use fmt print_str for uint64_t
