Access services in state test via public methods

[vkuznecovas]

This ensures that no races occur as all public API is behind a lock.

Fixes #2197

[codecov-commenter]

Codecov Report

Merging #2276 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2276      +/-   ##
==========================================
+ Coverage   47.87%   47.89%   +0.02%     
==========================================
  Files         273      273              
  Lines       12767    12767              
==========================================
+ Hits         6112     6115       +3     
+ Misses       6065     6061       -4     
- Partials      590      591       +1     

Impacted Files	Coverage Δ
nat/traversal/pinger.go	72.33% <0.00%> (-1.95%)	⬇️
core/connection/manager.go	82.36% <0.00%> (+0.46%)	⬆️
session/pingpong/invoice_tracker.go	67.86% <0.00%> (+0.65%)	⬆️
tequilapi/endpoints/sse_handler.go	74.78% <0.00%> (+2.60%)	⬆️

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