Improve tests using `rstest`

[thvdveld]

rstest provides useful macros for making tests more generic. A place where this is useful is in the interface. Here, we need to test for different mediums: IP, Ethernet and IEEE802.15.4. Previously, we had a variable MEDIUM which was the medium that was used by the interface, and depending on the feature flags that were set at compile time a different medium was used. This was not ideal. I think rstest is a better fit for achieving the same goal. I think this is a first step into making the writing of tests simpler, such that we can increase the coverage by adding more tests.

[codecov[bot]]

Codecov Report

Merging #796 (6239609) into master (a4cee91) will decrease coverage by 0.71%. The diff coverage is 25.39%.

@@            Coverage Diff             @@
##           master     #796      +/-   ##
==========================================
- Coverage   80.27%   79.57%   -0.71%     
==========================================
  Files          72       71       -1     
  Lines       29280    27653    -1627     
==========================================
- Hits        23505    22005    -1500     
+ Misses       5775     5648     -127     

Impacted Files	Coverage Δ
src/iface/interface/mod.rs	49.49% <7.31%> (-0.55%)	:arrow_down:
src/iface/interface/igmp.rs	50.57% <22.22%> (+14.48%)	:arrow_up:
src/iface/interface/sixlowpan.rs	74.93% <60.00%> (+4.08%)	:arrow_up:
src/iface/interface/ieee802154.rs	88.75% <100.00%> (+27.99%)	:arrow_up:
src/socket/udp.rs	84.88% <100.00%> (ø)

... and 8 files with indirect coverage changes

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[thvdveld]

~For some reason, the test directory does not count anymore for coverage. This file had a coverage of 100%. This is now gone. Overall, coverage increased.~

https://github.com/taiki-e/cargo-llvm-cov/pull/251 does not count test files for the coverage.

Overall, the coverage has increased for IPv6/IPv4/IEEE8021.5.4/6LoWPAN.