test: use pytest fixtures and os.devnull fixes

[TrueFurina]

Summary

438 test files decorate their unittest.TestCase classes with
@pytest.mark.usefixtures (no arguments). With no fixture names, the marker
is a no-op — pytest flags it on every run:

PytestWarning: usefixtures() in test/... without arguments has no effect

That produced ~1,500 PytestWarnings across the test suite. This PR removes
the no-op decorator line from all 438 files. Zero behavior change — the
classes are unittest.TestCase subclasses and no fixtures are involved.

Verification

• pytest test/unit before: 1529 passed, 1 failed, 30 skipped, 1501 warnings
• pytest test/unit after: 1529 passed, 1 failed, 30 skipped, 0 warnings
  (the 1 remaining failure, test_resolve_host6_should_return_a_list, is a
  DNS-network-dependent test unrelated to this change — it fails identically
  without the change on this machine)
• grep -c "usefixtures()" test/ → 0 remaining

Files changed

• 438 files under test/ (one decorator line removed each)