Add unit tests for db scripts

[lauravuo]

Purpose

• Add missing unit tests for db add and fetch scripts.
• Include unit tests for scripts to coverage measurements.
• Tests are run with firebase emulator.

Related Issues

Closes #24

How to test

See that CI passes or run locally:

yarn
yarn test:db

[codecov]

Codecov Report

Merging #34 (f3e2cb6) into main (4a71a8e) will increase coverage by 1.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #34      +/-   ##
==========================================
+ Coverage   97.77%   99.00%   +1.23%     
==========================================
  Files           7        8       +1     
  Lines          45      101      +56     
  Branches       11       19       +8     
==========================================
+ Hits           44      100      +56     
  Partials        1        1              

Flag	Coverage Δ
db-unittests	100.00% <100.00%> (?)
ui-unittests	97.77% <ø> (?)

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Impacted Files	Coverage Δ
tools/firebase/db.ts	100.00% <100.00%> (ø)

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[eevajonnapanula]

Looking good 🚀