test: cover source-specific membership on closed sockets

[Trott]

There are no other tests currently that confirm addSourceSpecificMembership() throws on a closed socket.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.43%. Comparing base (dd9777b) to head (2706a44).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66624      +/-   ##
==========================================
- Coverage   92.78%   90.43%   -2.36%     
==========================================
  Files         422      791     +369     
  Lines      193692   276585   +82893     
  Branches    29881    53110   +23229     
==========================================
+ Hits       179714   250121   +70407     
- Misses      13650    16869    +3219     
- Partials      328     9595    +9267     

see 499 files with indirect coverage changes

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