Let the test battery honor DATALAD_TESTS_NONETWORK

[mih]

This is primarily achieved via a custom pytest mark. However, there is a session-level fixture that itself depends on network access. Unlike with a function-level fixture, I cannot figure out how to make any dependent test be skipped. Instead, I need to use the mark as a skipping decorator.

With these changes I can successfully run the test suite on a system with no network access (not just a proxy-based diversion):

247 passed, 39 skipped, 19 warnings in 171.10s (0:02:51)

Ping #490

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (33b8f51) 92.41% compared to head (5dfa5f6) 92.46%.
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #504      +/-   ##
==========================================
+ Coverage   92.41%   92.46%   +0.04%     
==========================================
  Files         126      127       +1     
  Lines        9577     9593      +16     
  Branches     1036     1037       +1     
==========================================
+ Hits         8851     8870      +19     
+ Misses        704      700       -4     
- Partials       22       23       +1     

Files	Coverage Δ
...alad_next/archive_operations/tests/test_tarfile.py	100.00% <100.00%> (ø)
...lad_next/commands/tests/test_ls_file_collection.py	100.00% <100.00%> (ø)
...atalad_next/iter_collections/tests/test_itertar.py	100.00% <100.00%> (ø)
..._next/patches/tests/test_annex_progress_logging.py	100.00% <100.00%> (ø)
datalad_next/tests/marker.py	100.00% <100.00%> (ø)
datalad_next/url_operations/tests/test_http.py	60.43% <100.00%> (+0.78%)	⬆️
datalad_next/tests/fixtures.py	93.42% <50.00%> (-0.58%)	⬇️

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.