Fix socket tests on older Linux kernels

[de11n]

Ran into this trying to run the tests on RHEL8.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00000%. Comparing base (578efcb) to head (72b49bb).
Report is 2 commits behind head on main.

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3219   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             124          124           
  Lines           18772        18772           
  Branches         1269         1269           
===============================================
  Hits            18772        18772           

Files with missing lines	Coverage Δ
src/trio/_tests/test_socket.py	100.00000% <100.00000%> (ø)

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[jakkdl]

lgtm! Minimal fix, great comment ✨

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