ci: fail if any Go files contain an ignore directive

[marten-seemann]

I almost accidentally committed a test file containing an ignore directive. This wouldn't have been caught by CI, the code would just have been untested.

[codecov]

Codecov Report

Merging #4055 (7cfc04d) into master (8f34488) will increase coverage by 0.02%.
Report is 1 commits behind head on master.
The diff coverage is n/a.

❗ Current head 7cfc04d differs from pull request most recent head decea66. Consider uploading reports for the commit decea66 to get more accurate results

@@            Coverage Diff             @@
##           master    #4055      +/-   ##
==========================================
+ Coverage   83.27%   83.29%   +0.02%     
==========================================
  Files         147      144       -3     
  Lines       14908    14859      -49     
==========================================
- Hits        12414    12376      -38     
+ Misses       1993     1985       -8     
+ Partials      501      498       -3     

see 7 files with indirect coverage changes