Don't create IPv6 socket if IPv6 is disabled

[jackwilsdon]

test_socket.py:test_sniff_sockopts creates an IPv6 socket even if IPv6 is disabled. This PR changes the test to only create the socket if IPv6 is available.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.62%. Comparing base (785dc59) to head (6995a38).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/trio/_tests/test_socket.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3136      +/-   ##
==========================================
- Coverage   99.62%   99.62%   -0.01%     
==========================================
  Files         122      122              
  Lines       18357    18360       +3     
  Branches     1221     1222       +1     
==========================================
+ Hits        18289    18291       +2     
  Misses         47       47              
- Partials       21       22       +1     

Files with missing lines	Coverage Δ
src/trio/_tests/test_socket.py	99.84% <75.00%> (-0.16%)	⬇️

[A5rocks]

This makes sense to me. Did you run into this when running trio's test suite and this was the only issue?

[jackwilsdon]

Yeah all other tests passed with IPv6 disabled 👍

[CoolCat467]

Looks good

[A5rocks]

Made #3137 as a follow-up.

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