tests: add unit tests

[filipstefansson]

What this PR is about:

• Add unit tests and code coverage

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (alpha@f500927). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             alpha        #7   +/-   ##
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  Coverage         ?   100.00%           
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  Files            ?         4           
  Lines            ?        49           
  Branches         ?        14           
=========================================
  Hits             ?        49           
  Misses           ?         0           
  Partials         ?         0           

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[github-actions[bot]]

:tada: This PR is included in version 1.0.0-alpha.5 :tada:

The release is available on:

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

Your semantic-release bot :package::rocket:

[github-actions[bot]]

:tada: This PR is included in version 1.0.0 :tada:

The release is available on:

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

Your semantic-release bot :package::rocket: