Fix SCTPTransport OnClose test

[sukunrt]

We need to close both the ends here for all the goroutines to complete.

The corresponding commit in v3 is here: 484d89a

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.90%. Comparing base (3aab035) to head (0805116).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2864      +/-   ##
==========================================
+ Coverage   78.85%   78.90%   +0.04%     
==========================================
  Files          89       89              
  Lines        8452     8452              
==========================================
+ Hits         6665     6669       +4     
+ Misses       1299     1296       -3     
+ Partials      488      487       -1     

Flag	Coverage Δ
go	80.46% <ø> (+0.05%)	⬆️
wasm	65.11% <ø> (ø)

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