[tests] use base64 for json to prevent it showing up in grep

[codecov-io]

Codecov Report

Merging #65 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #65   +/-   ##
=======================================
  Coverage   52.16%   52.16%           
=======================================
  Files          36       36           
  Lines        3715     3715           
=======================================
  Hits         1938     1938           
  Misses       1580     1580           
  Partials      197      197

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[tmc]

done.