Use randomly-available port in otlp exporters tests

[dmathieu]

These tests are randomly failing quite frequently (See https://github.com/open-telemetry/opentelemetry-go/runs/5252854681 for example).
As far as I can see, the TCP listener is always closed properly. So I'm assuming this may be due to tests running in parallel (maybe across runs?).

By using random ports, we guarantee we will always have a free port, which should remove the test flakiness.

[codecov]

Codecov Report

Merging #2627 (2ba3c7d) into main (8ba8f68) will increase coverage by 0.0%.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main   #2627   +/-   ##
=====================================
  Coverage   76.1%   76.1%           
=====================================
  Files        173     173           
  Lines      12234   12234           
=====================================
+ Hits        9318    9322    +4     
+ Misses      2673    2669    -4     
  Partials     243     243           

Impacted Files	Coverage Δ
sdk/trace/batch_span_processor.go	82.1% <0.0%> (+1.8%)	⬆️

[XSAM]

👍