pytest: tracking porting efforts of release tests

[miri64]

This is a tracking PR, tracking all the porting efforts of the release tests and it can be used to run all the tests as a whole.

• Base implementation (testutils: Automated testing environment for release tests using pytest #155)
• Spec 3 (03-single-hop-ipv6-icmp: initial import of automated tests #160)
• Spec 4 ( 04-single-hop-6lowpan-icmp: initial import of automated tests #161)
• Spec 5 (05-single-hop-route: initial import of automated tests #162)
• Spec 6 (06-single-hop-udp: initial import of automated tests #163)
• Spec 7 (07-multi-hop: port automated tests to pytest  #158)
• Spec 8 (08-interop: initial import of automated tests #159)
• Spec 9 (09-coap: port to pytest-based framework #168)
• Spec 10 (10-icmpv6-error: initial import of automated tests #167)

[miri64]

There is also this branch that integrates the release tests with Github Actions (currently only running the tests that run on native as there are no IoT-LAB credentials integrated yet).

[miri64]

With #168 the last spec I wanted to port to the test framework is done. Depending on what the Github Action running the tests now says, I consider this done.

[miri64]

(apart from the help wanted in #168 of course)

[miri64]

Rebased to current master and dependencies.

[miri64]

Rebased to current master and dependencies.

[miri64]

Rebased to current master and dependencies.

[miri64]

Rebased to current master and dependencies.

[miri64]

All test specs are now merged. I moved the default activation of flake8 and pylint in tox to #179 now.