feat: enable autobahn tests against external test server

[mccutchen]

The idea here is that there are some test cases (e.g. in the 9.* suite that send extremely large frames/messages) where our implementation appears to be painfully slow, so being able to run specific autobahn tests against an external server like

make testautobahn AUTOBAHN_CASES=9.2.6 AUTOBAHN_TARGET=localhost:8080

will hopefully make it easier to gain visibility into performance issues. (See also #23 for one potential path towards optimization.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.62%. Comparing base (95b67c2) to head (f9518f3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
+ Coverage   79.53%   79.62%   +0.09%     
==========================================
  Files           3        3              
  Lines         430      432       +2     
==========================================
+ Hits          342      344       +2     
  Misses         64       64              
  Partials       24       24              

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