log/loki: harden client test

[gsora]

In case of slow httptest handler execution, don't block and provoke client retransmission, which would result in entriesChan to have two "normal log line" entries.

category: bug
ticket: none

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.11% ⚠️

Comparison is base (97b946a) 53.77% compared to head (a164b5d) 53.66%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2525      +/-   ##
==========================================
- Coverage   53.77%   53.66%   -0.11%     
==========================================
  Files         199      199              
  Lines       26908    26908              
==========================================
- Hits        14469    14441      -28     
- Misses      10645    10663      +18     
- Partials     1794     1804      +10     

Files Changed	Coverage Δ
app/log/loki/client.go	68.06% <100.00%> (-12.61%)	⬇️

... and 10 files with indirect coverage changes

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