08-interop: deprecate Contiki tests in favor of Contiki-NG

[jia200x]

Contribution description

This PR deprecates the Contiki test in favor of Contiki-NG. The original test is hard to run and requires the CC2538DK evaluation board, which costs ~500 USD and only a few maintainers have access to it.

The new test consist of ICMPv6 exchange between a RIOT node and a Contiki-NG node using link-local addresses. Contiki-NG is easy to compile and flash, as the toolchain and flashers run inside Docker.

Testing procedure

• Follow the instructions of the new Contiki-NG test (08-interop: Task 03).
• Make sure there aren't any format issues in the new description.

Issues/PRs references

279

[jia200x]

amended directly!

[MrKevinWeiss]

superseded by #292