Improvement/improve specs

[frontendphil]

• remove all those callbacks from the specs
• use @testing-library/react in specs
• add flow-types to all specs

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[codecov-commenter]

Codecov Report

Merging #152 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #152   +/-   ##
=======================================
  Coverage   96.09%   96.09%           
=======================================
  Files          13       13           
  Lines         282      282           
=======================================
  Hits          271      271           
  Misses         11       11           

Impacted Files	Coverage Δ
src/StickNode.js	100.00% <ø> (ø)
src/Stick.js	92.75% <100.00%> (ø)

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[workflow-bot]

🎉 This PR is included in version 4.1.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀