[CI] Add static port for observer bind address in integration tests

[peterargue]

Occasionally, observers fail to start because of a port conflict while starting the network (Run):

container.go:219: testingdock: container start failure: Error response from daemon: driver failed programming external connectivity on endpoint observer_1 (eea0760122c76ba24b88f943b71079c312b5354c5d79fe3a309d5000a56de93f): Bind for 0.0.0.0:43343 failed: port is already allocated

There is likely a race condition/bug within testingdock where it doesn't synchronize the ports it allocates with those the OS allocates. Setting a fixed bind port should avoid this situation.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 55.69%. Comparing base (ed9f53f) to head (44e1252).

Files	Patch %	Lines
integration/testnet/network.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5801      +/-   ##
==========================================
+ Coverage   54.66%   55.69%   +1.03%     
==========================================
  Files         942     1094     +152     
  Lines       75331    85553   +10222     
==========================================
+ Hits        41177    47646    +6469     
- Misses      30089    33301    +3212     
- Partials     4065     4606     +541     

Flag	Coverage Δ
unittests	55.69% <0.00%> (+1.03%)	⬆️

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