tests/test_netns: define test dependencies

[Bastian-Krause]

Description
Instead of letting the tests fail, define what is required to run them. Skip if the requirements are not met.

Also add requests[socks] to the labgrid[dev] extra to allow tests/test_netns.py::test_socks to run.

Checklist

• PR has been tested (on top of Fix issues that prevent using RawNetworkInterfaceDriver's network namespace feature #1887 to prevent the "Operation not permitted" error, but this PR does not depend on that)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.0%. Comparing base (915b591) to head (f155b06).
⚠️ Report is 61 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #1938      +/-   ##
=========================================
+ Coverage    46.0%   61.0%   +15.0%     
=========================================
  Files         180     182       +2     
  Lines       14464   14881     +417     
=========================================
+ Hits         6654    9083    +2429     
+ Misses       7810    5798    -2012     

Flag	Coverage Δ
3.10	61.0% <ø> (+15.0%)	⬆️
3.11	61.0% <ø> (+15.0%)	⬆️
3.12	61.0% <ø> (+15.0%)	⬆️
3.13	60.9% <ø> (+15.0%)	⬆️
3.14	60.9% <ø> (+15.0%)	⬆️

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