Make encoding tests independent of LOCALE settings

[guillep2k]

I've noticed that the encoding tests I've added in #7814 have failed on the drone with an unrelated PR (see this run).

My suspicion is that the drone is running with a LOCALE different from en_US, so this PR is an attempt to make the tests independent from the compilation locale.

[codecov-io]

Codecov Report

Merging #8018 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #8018      +/-   ##
==========================================
+ Coverage   41.57%   41.57%   +<.01%     
==========================================
  Files         480      480              
  Lines       64144    64144              
==========================================
+ Hits        26669    26671       +2     
+ Misses      34015    34013       -2     
  Partials     3460     3460

Impacted Files	Coverage Δ
models/repo_indexer.go	64.63% <0%> (-2.04%)	⬇️
modules/log/event.go	64.61% <0%> (-1.03%)	⬇️
routers/repo/view.go	43.54% <0%> (+1.01%)	⬆️
modules/log/log.go	72.07% <0%> (+2.7%)	⬆️
models/unit.go	67.56% <0%> (+5.4%)	⬆️

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