conntest: Add 100% coverage

- conntest: Small coherency fixes along the way.
- conntest: Add Discard. Useful for zero filled tests.
- reg: Improve testing.
- CONTRIBUTING.md: fix a typo.

CONTRIBUTING.md

@@ -1,4 +1,4 @@
 # Contributing
 
-Thanks for contributing to the project! Please look at [periph the
+Thanks for contributing to the project! Please look at [the periph contribution
-contribution guidelines](https://periph.io/project/contributing/) first.
+guidelines](https://periph.io/project/contributing/) first.