WIP: Fold the PR comment when tests are passing

[JasonEtco]

Error logs are big and verbose which is extremely helpful, but only until the build is passing again. ci-reporter should hide the previous error logs when a PR is passing.

• Update comment when build is passing
• Ensure that when build fails again, the comment still works
• Write tests

Closes #34

[codecov]

Codecov Report

Merging #41 into master will decrease coverage by 9.32%.
The diff coverage is 38.88%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
- Coverage     100%   90.67%   -9.33%     
==========================================
  Files           7        8       +1     
  Lines         102      118      +16     
  Branches       10       14       +4     
==========================================
+ Hits          102      107       +5     
- Misses          0        8       +8     
- Partials        0        3       +3

Impacted Files	Coverage Δ
src/get-existing-comment.js	100% <100%> (ø)
src/index.js	76.08% <21.42%> (-23.92%)	⬇️

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