[chore] apply some nits to contract_checker.

[atoulme]

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (59b45a5) 90.61% compared to head (68ab660) 90.63%.

Files	Patch %	Lines
receiver/receivertest/contract_checker.go	80.00%	0 Missing and 2 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9543      +/-   ##
==========================================
+ Coverage   90.61%   90.63%   +0.01%     
==========================================
  Files         347      347              
  Lines       18236    18230       -6     
==========================================
- Hits        16525    16522       -3     
+ Misses       1381     1379       -2     
+ Partials      330      329       -1     

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